Satellite images appear to show movement at North Korea's main nuclear site, which could be associated with the reprocessing of radioactive material into bomb fuel.A US think tank reported that satellite pictures from Yongbyon nuclear site from 12 April showed five specialised railcars near its Uranium Enrichment Facility and Radiochemistry Laboratory.The think tank, Beyond Parallel, said the movement could indicate the transfer of radioactive material.Image:Researchers have said this is the Radiochemistry Laboratory at the Yongbyon Nuclear Research CenterThe report said: "In the past, these specialised rail cars appear to have been associated with the movement of radioactive material or reprocessing campaigns."The current activity, along with their configurations, does not rule out their possible involvement in such activity, either before or after a reprocessing campaign."Any new reprocessing activity would underscore the failure of the second summit between US President Donald Trump and Kim Jong Un in Hanoi in February.The summits have been intended to denuclearise North Korea.Image:Kim Jong Un and President Trump have met twice to discuss denuclearisationJenny Town, a North Korea expert at Stimson Centre think tank said reprocessing would be a significant development given US-North Korea talks.She said: "Because there wasn't an agreement with North Korea on Yongbyon, it would be interesting timing if they were to have started something so quickly after Hanoi."Talks in Hanoi collapsed after Mr Trump proposed a "big deal" in which sanctions would be lifted on the secretive nation.

But he refused promises of partial denuclearisation.Image:Beyond Parallel describe these as specialised rail cars Although Mr Kim has maintained a freeze in missile and nuclear tests since 2017, US officials say North Korea has continued to produce fissile material that can be processed for use in bombs.Last month, a senior North Korean official warned that Mr Kim might rethink the test freeze unless the US made concessions.Last week, Mr Kim said the Hanoi breakdown raised risks of reviving tensions, adding he was only interested in meeting Mr Trump again if the US came with the right attitude.He said he would "wait til the end of the year" for the US to decide to be more flexible.Image:Researchers of Beyond Parallel think this is a 20-foot shipping container near the uranium enrichment plant On Monday, Mr Trump and Mike Pompeo brushed aside the demand.

Mr Pompeo, secretary of state, said Mr Kim should keep his promise to give up his nuclear weapons before then.Ms Town said new reprocessing work at Yongbyon would show the importance of the facility, and added: "It would underscore that it is an active facility that does increase North Korea's fissile material stocks to increase its arsenal."Experts say North Korea's arsenal is anywhere between 20 and 60 warheads.