According to US intelligence chief James Clapper, North Korea has restarted a plutonium production reactor that could provide a stockpile for nuclear weapons.
James Clapper also said North Korea had taken steps towards making an intercontinental ballistic missile system.
His announcement comes days after North Korea launched a long-range rocket, which critics say is a test of banned missile technology.
In September 2015, Pyongyang said its main nuclear facility at Yongbyon had resumed normal operations.
The reactor there has been the source of plutonium for its nuclear weapons program.
North Korea carried out its fourth nuclear test on January 6, 2016.
“We assess that North Korea has followed through on its announcement by expanding its Yongbyon enrichment facility and restarting the plutonium production reactor,” James Clapper told the Senate Armed Services Committee.
“We further assess that North Korea has been operating the reactor long enough so that it could begin to recover plutonium from the reactor’s spent fuel within a matter of weeks to months.”
According to James Clapper, Pyongyang was also committed to developing a long-range, nuclear-armed missile “capable of posing a direct threat to the United States”.
James Clapper said it had publicly displayed a mobile intercontinental ballistic missile system and had taken “initial steps toward fielding this system, although the system has not been flight-tested”.
Experts have said that, when fully operational, the Yongbyon reactor could make one nuclear bomb’s worth of plutonium per year. About 4kg of plutonium is needed in order to make a bomb that would explode with a force of 20 kilotons.
Pyongyang has pledged several times to stop operations at Yongbyon and even destroyed the cooling tower in 2008 as part of a disarmament-for-aid deal.
In March 2013, following a row with the US and with new UN sanctions over a third nuclear test, it vowed to restart all facilities at Yongbyon.
Six-nation talks involving South Korea, the US, China, Japan and Russia aimed at ending the North’s nuclear programme have been stalled since 2009.
North Korea says it has made a device small enough to fit a nuclear warhead on to a missile, which it could launch at its enemies. However, US officials have cast doubt on the claim.
North Korea may have restarted one of its nuclear reactors, satellite pictures taken earlier this year suggest.
According to the Institute for Science and International Security, images of the Yongbyon plant show patterns of melting snow indicating new activity.
Yongbyon’s reactor was shut down in 2007 but was restarted in 2013.
Six-nation talks aimed at ending North Korea’s nuclear program have been stalled since early 2009.
The think-tank’s report says that its assessment in late 2014 was that the reactor at Yongbyon, in the country’s west, had been “shut down or partially shut down” but more recent images suggested the plant “may be operating at low power or intermittently”.
As well as the patterns of melting snow, the satellite photos are also said to capture a stream of warm water coming out of the reactor’s discharge pipeline and steam rising off the turbine.
In 2008, North Korea walked away from six-party talks with South Korea, the US, China, Japan and Russia on its denuclearization.
The other countries have since tried to persuade North Korea to return to the negotiating table.
Earlier this year North Korea offered to stop nuclear tests if the US stopped holding military drills with Seoul, but the offer was rejected.
North Korea has conducted three nuclear tests, in 2006, 2009 and 2013.
According to a US institute, steam has been seen rising from North Korea’s Yongbyon nuclear facility, suggesting that the reactor has been restarted.
The color and volume of the steam indicated that the reactor was in or nearing operation, the institute said.
Pyongyang vowed to restart facilities at its main Yongbyon nuclear complex in April, amid high regional tensions.
The reactor can produce plutonium, which North Korea could use to make nuclear weapons.
Analysts believe North Korea already possesses between 4 and 10 nuclear weapons, based on plutonium produced at the Yongbyon reactor prior to mid-2007, when the facility was closed down.
The report, which was published on the 38 North website on Wednesday, was written by the US-Korea Institute at Johns Hopkins University. The institute uses satellite imagery to monitor developments in North Korea.
Pyongyang vowed to restart facilities at its main Yongbyon nuclear complex in April, amid high regional tensions
The reactor uses steam turbines to generate electricity, and the steam seen in satellite imagery from August 31 indicated that the electrical system was about to come online, the report said.
“The reactor looks like it either is or will within a matter of days be fully operational, and as soon as that happens, it will start producing plutonium,” said report author Jeffrey Lewis.
“They really are putting themselves in a position to increase the amount of material they have for nuclear weapons, which I think gives them a little bit of leverage in negotiations, and adds a sense of urgency on our part,” he added.
The five megawatt reactor can produce spent fuel rods that can be made into plutonium, which experts believe North Korea used for its nuclear tests in 2006 and 2009. North Korea conducted its third, most-recent test in February, but it is not clear whether plutonium or uranium was used.
In a November 2010 reportfollowing a visit to Yongbyon, US scientist Siegfried Hecker said that based on what he saw he believed North Korea could “resume all plutonium operations within approximately six months” at Yongbyon, then shut down, if so inclined.
Analysts at the Institution for Science and International Security, a think tank, said it would take a considerable amount of time before North Korea could use any new plutonium in nuclear weapons.
“Given that North Korea will likely need two-three years before it discharges irradiated fuel containing plutonium and another six to 12 months to separate the plutonium, there remains time to negotiate a shutdown of the reactor before North Korea can use any of this new plutonium in nuclear weapons,” it said in a report.
Analysts say the reactor can produce six kg (13 lbs) of plutonium a year – enough to make one to two nuclear bombs.
Both the US State Department and South Korea’s National Intelligence Service have declined to comment directly on the report, saying they do not comment on intelligence matters, AP news agency reported.
North Korea closed the Yongbyon reactor in July 2007 as part of a disarmament-for-aid deal.
The cooling tower at the facility was later destroyed, but then the disarmament deal stalled, partly because the US did not believe Pyongyang was fully disclosing all of its nuclear facilities.
In 2010, North Korea unveiled a uranium enrichment facility at Yongbyon to Siegfried Hecker.
Siegfried Hecker said that while the facilities appeared to be for electricity generation purposes, it could be readily converted to produce highly-enriched uranium for bombs.
North Korea is reactivating facilities at its moth-balled Yongbyon nuclear reactor, a US think-tank says.
Start-up could be one to two months away, it said, but there was uncertainty over the availability of fuel rods to power the reactor.
Pyongyang vowed to restart the reactor, which makes weapons-grade plutonium, in April amid severe regional tensions.
The Yongbyon reactor was shut down in July 2007 as part of a disarmament-for-aid deal.
The cooling tower at the facility was later destroyed, but then the disarmament deal stalled.
North Korea’s decision to restart followed its third nuclear test on February 12, which led to expanded UN sanctions.
The information came from the 38 North website, which is part of the US-Korea Institute at Johns Hopkins University in the US.
Its report said that recent satellite imagery showed that North Korea had “essentially finished repairing the cooling system necessary to restart and operate the reactor”.
North Korea is reactivating facilities at its moth-balled Yongbyon nuclear reactor
The cooling tower that was destroyed had not been repaired, but instead a secondary cooling system had been employed. Work was also ongoing at a spent fuel facility, it said.
Piles of construction materials were visible at the site and what could be a new drainage ditch for water from the reactor building was being dug, it said.
The reactor “may be one to two months from start-up. However, the availability of fresh fuel rods to power the reactor – a key factor that will determine when the North will restart the facility – remains unclear,” it said.
Once operational, the reactor could produce “approximately six kilograms of plutonium per year that can be used for manufacturing nuclear weapons”, it added.
North Korea has conducted three nuclear tests since 2006. Analysts believe the first two tests used plutonium as the fissile material, but it is not known whether the third used plutonium or uranium.
While North Korea has depleted its stocks of “reactor-grade” plutonium needed to make the weapons-grade variety, it has plentiful reserves of uranium ore. It also has a uranium enrichment facility at Yongbyon which a US scientist said could be converted to produce highly enriched uranium bomb fuel.
After UN sanctions were expanded following the most recent nuclear test, in February, North Korea issued multiple threats against US and regional interests, vowed to reactivate Yongbyon and cut both official communications and key business ties with South Korea.
Operations at the jointly-run inter-Korean Kaesong industrial zone remain suspended – the first such stoppage since the project began.
But the threats have diminished in recent weeks and last month, North Korea sent a top envoy to Beijing – its first such move since its nuclear test.
Later this week, the US and Chinese presidents meet in California for their first summit, with North Korea likely to be high on the agenda.