The US has given Turkey an ultimatum to choose between buying US fighter jets and Russian anti-aircraft missile systems by the end of July.
Acting Defense Secretary Patrick Shanahan set out the deadline in a letter to his Turkish counterpart, Hulusi Akar.
Turkey, Patrick Shanahan said, could not have both America’s F-35 advanced fighter jets and Russia’s S-400 systems.
The two NATO allies have been locked in a row over the S-400 for months.
The US argues that the Russian systems are both incompatible with NATO defense systems and pose a security threat, and wants Turkey to buy its Patriot anti-aircraft systems instead.
Turkey, which has been pursuing an increasingly independent defense policy, has signed up to buying 100 F-35s, and has invested heavily in the F-35 program, with Turkish companies producing 937 of the plane’s parts.
Patrick Shanahan says in his letter that the US is “disappointed” to hear that Turkish personnel have been sent to Russia to train on the S-400.
“Turkey will not receive the F-35 if Turkey takes delivery of the S-400,” he writes.
“You still have the option to change course on the S-400.”
Kim Jong-un has said North Korea is close to testing long-range missiles capable of carrying nuclear warheads.
During his New Year’s message, Kim Jong-un claimed that the intercontinental ballistic missiles were in their “last stage” of development.
North Korea has conducted two nuclear tests in 2016, including its biggest one to date.
This raised fears that Pyongyang has made significant nuclear advances.
However, it has never successfully test-fired such a missile.
Reuters reported a senior US military official as saying that although Pyongyang appears able to put a miniaturized nuclear warhead on a missile, the missile re-entry technology necessary for longer range strikes is still a serious obstacle to its weapons development.
Kim Jong-un, who took control of the secretive state following his father’s death in 2011, said during a TV addresss: “Research and development of cutting edge arms equipment is actively progressing and ICBM [inter-continental ballistic missile] rocket test launch preparation is in its last stage.”
The north Krean leader said his country was now a “military power of the East that cannot be touched by even the strongest enemy”.
UN resolutions call for an end to North Korea’s nuclear and missile tests.
When Pyongyang tested its nuclear bomb in September 2016, estimates varied on how strong it was.
The September test triggered widespread condemnation and further international sanctions against North Korea.
South Korea’s defense ministry and Japanese media say North Korea appears to be preparing to launch a long-range missile.
Activity has been spotted at a launch station on the North Korea’west coast of the isolated nation.
Earlier this week Pyongyang announced it was planning to launch a satellite at some point in February.
North Korea’s announcement was internationally condemned – critics saying it is a cover to test banned missile technology.
The isolated country also conducted its fourth nuclear bomb test on January 6.
UN sanctions against North Korea prohibit it from carrying out any nuclear or ballistic missile tests.
South Korean state news agency Yonhap quoted defense ministry officials on February 4 as saying activity had been spotted at a site in Dongchang-ri, where the Sohae launching station is located.
Defense Ministry spokesman Moon Sang-gyun also said South Korea’s military was ramping up its air defense readiness so it was ready to intercept any missile or debris falling in its territory. The South has already ordered certain commercial flights to divert their routes.
Japan’s national broadcaster NHK, citing unnamed officials, also reported similar news about activity at Dongchang-ri, and added that a mobile launcher carrying a ballistic missile had also been seen moving near the east coast.
Separately, South Korean President Park Geun-hye said in a statement reported by Yonhap that any long-range missile launch by the North “should never be condoned as it poses a threat to peace on the Korean Peninsula and the world”.
Park Geun-hye said the move was “a desperate measure” by the North to maintain its regime, and showed Pyongyang was not afraid of UN sanctions.
The US-based North Korean analysis website 38 North said recent satellite images show recent activity at Sohae suggesting launch preparations.
These include heightened activity at a building used to receive rocket stages, and a complex that appears ready to conduct engine tests.
North Korean state news agency KCNA reported on February 4 that the ruling Workers’ Party of Korea was holding a meeting among central and army committee members where they discussed how to “further strengthen” the party ahead of a rare political meeting scheduled for May.
Analysts say North Korea’s recent nuclear and missile activity could be a build-up to the upcoming seventh party congress – the first to be held since 1980 – where leader Kim Jong-un is expected to show off the nuclear program.