SEOUL • North Korea has completed an upgrade of its main long-range rocket site, a sign it may be preparing for a launch, forbidden under United Nations sanctions, according to a group that watches the country's military developments.

North Korea can now set up a long-range rocket at its Sohae launch tower, after preparing its stages and payload at a new support building, said 38 North, a Johns Hopkins University website, in an e-mailed report, citing commercial satellite images.

"In the coming weeks, if preparations are indeed under way, we would expect to see other on-the-ground indications at Sohae," 38 North said.

Such preparations would include transferring equipment, filling oxidiser and fuel storage tanks, and political leaders arriving at the site to observe a launch.

North Korea last used the Sohae site on the country's western coast in 2012, launching a rocket that flew thousands of kilometres and put a satellite into orbit.

The United States and South Korea condemned the launch as a test of a ballistic missile banned under UN Security Council resolutions.

While North Korea has not obtained the ability to equip a long-range missile with a nuclear warhead, the Kim Jong Un regime has made "considerable progress" in its efforts, South Korean officials have said.

Any proof that North Korea is capable of launching a nuclear-tipped missile could lend more urgency to efforts to restart aid-for-disarmament talks, stalled since 2008.

The US remains open to talks with North Korea while the Kim regime continues to ignore calls for dialogue, US nuclear envoy Sydney Seiler told reporters in Seoul on Monday.

A day later, North Korea's ambassador to China, Mr Ji Jae Ryong, told reporters in Beijing that his country will continue to bolster its nuclear capabilities, blaming the US for tensions.

BLOOMBERG