US experts say North Korea appears to be upgrading one of its two rocket launch sites, perhaps in a move to test bigger rockets.
“Important progress” had been made at Tonghae Satellite Launching Ground since October 2012, the analysis from the 38 North website said.
Activities around the new launch pad also revealed possible evidence of assistance from Iran, it said.
Pyongyang used a three-stage rocket to put a satellite into space last year.
That launch – condemned by the UN as a banned test of missile technology – took place at the Sohae launch site.
But previous unsuccessful attempts in 2006 and 2009 took place at the Tonghae site, which is also known as Musudan-ri.
The analysis from 38 North, the website of the US-Korea Institute at Johns Hopkins School of Advanced International Studies in Baltimore, was based on satellite imagery.
Construction of the new launch pad was continuing, it said, with images showing Pyongyang would be able to test rockets “perhaps three to four times the size of the Unha [launched in December 2012] when construction is completed, possibly in 2016”.
Two new design features were similar to those used at the Semnan Launch Complex in Iran, it said.
The images also confirmed activity at the old launch pad.
US experts say North Korea appears to be upgrading one of its two rocket launch sites, perhaps in a move to test bigger rockets
“That activity may be related to another round of modifications intended to support future launches of the Unha rocket or possibly another liquid-fuelled missile,” 38 North said, while cautioning that more information was needed.
North Korea last week conducted its third nuclear test, claiming to have successfully detonated a smaller but more powerful device than in previous tests.
The move drew immediate condemnation from the UN Security Council.
Observers fear North Korea is working towards creating a nuclear device small enough to fit on a long-range missile.
According to South Korean officials, North Korea’s recent rocket launch shows it has the ability to fire a rocket more than 10,000 km (6,200 miles).
The estimate, which would potentially put the Western US in range, was based on an analysis of rocket debris.
However, there was no confirmation that the North had the re-entry technology needed to deliver a missile.
Experts believe North Korea is also years away from gaining the ability to mount a nuclear bomb on a missile.
North Korea launched the Unha-3 rocket on December 12, in defiance of sanctions and international warnings.
It was the first time the North had made successful use of a three-stage rocket to put a satellite into orbit, and observers said it appeared to mark a step towards fielding an intercontinental range ballistic missile.
“As a result of analyzing the material of Unha-3 [North Korea’s rocket], we judged North Korea had secured a range of more than 10,000km in case the warhead is 500-600kg,” a South Korean defence ministry official told journalists.
The official said the type of oxidizer container that was found from the first stage of the rocket launch would rarely be used by countries with advanced space technology.
“Welding was crude, done manually,” the official said.
North Korea’s recent rocket launch shows it has the ability to fire a rocket more than 6,200 miles
South Korea would not be able to tell whether the North had the technology to achieve re-entry until debris from the second and third stages of the rocket launch was analyzed, the defence ministry said.
“As the additional pieces are salvaged, we will be able to look deeper into the function and structure of North Korea’s long-range rocket,” an official was quoted as saying by South Korea’s Yonhap news agency.
Experts believe many more rocket and nuclear tests will be necessary before North Korea can boast a credible delivery system.
North Korea insists the rocket it launched is part of a civilian space programme.
The North has been happy declare itself a nuclear power and it frequently threatens neighboring countries, and the US, with massive retaliation for perceived slights.
The UN Security Council condemned this month’s rocket launch.
It said it violated two UN resolutions banning Pyongyang from missile tests, passed after it conducted nuclear tests in 2006 and 2009.
The rocket was celebrated extravagantly in North Korea, with a mass rally held in the capital, Pyongyang.
The North’s leader, Kim Jong-un, called for the development and launching of “a variety of more working satellites” and “carrier rockets of bigger capacity” at a banquet to mark the launch on Friday, North Korean state media reported.