Washington, Mar 6 (IANS): New satellite images of North Korea suggest it has begun rebuilding a portion of a facility previously used to test long-range missile engines, analysts have said, raising questions about the future of US-North Korea negotiations.

The images were taken two days after talks between US President Donald Trump and North Korea's Kim Jong-un ended on February 28, without reaching a deal on denuclearization. Pyongyang had earlier pledged to dismantle the site as a confidence-building measure with Washington.

The Centre for Strategic Studies' Beyond Parallel project and 38 North, both North Korea monitoring websites, said they observed activity at the Sohae site at Tongchang-ri satellite launch facility, dormant since about August of last year, CNN reported.

Tongchang-ri is reportedly one of the few known missile component development facilities inside North Korea.

According to 38 North, efforts to rebuild the site's launch pad and missile engine test stand began sometime between February 16 and March 2, meaning work began either in the days before, during, or immediately after Kim and Trump abruptly ended their second summit in Vietnam on February 28 without signing a deal.

Beyond Parallel reported there had been "activity" at a vertical engine test pad and a launch pad's rail-mounted rocket transfer structure. It said that the activity was "deliberate and purposeful".

The Central Intelligence Agency declined to comment on the photographs, CNN said.

The news came a day after the US warned that North Korea could face yet more sanctions if it not take steps to denuclearize.

In an interview with FOX Business Network, US National Security Adviser John Bolton said Washington would see whether the Pyonyang was still serious about the negotiations and was committed.

"If they're not willing to do it... they're not going to get relief from the crushing economic sanctions that have been imposed on them and we'll look at ramping those sanctions up in fact," Xinhua news agency quoted Bolton as saying.

Sohae has been North Korea's main satellite launch facility since 2012. It has also been used for testing engines for missiles capable of reaching the US. However, it has never been used for testing the ballistic missiles that are considered so provocative.

"This distinction is important," Jenny Town, Managing Editor of monitoring 38 North, told the BBC. "The North Koreans likely see the rebuilding not as an active part of their missile programme, but of their civilian space programme -- a distinction they have made repeatedly in the past.

"One would assume, the rebuilding of these structures signals a diminished lack of trust in the process," said Town.