"Washington thinks it is highly likely that those activities are a part of North Korea's operations to close down the nuclear facility," a source revealed to the publication.
Yonhap news agency also reported some unusual activities occurring around the plant but quoted an unnamed intelligence official who asserted that the nuclear site was still in operation.
The US has continued to urge the reclusive communist nation to shut down its Yongbyon plant as part of the deal reached in February in multilateral talks with neighbouring South Korea as well as Japan, China and Russia.
However the 60-day deadline expired on Saturday with North Korea reluctant to play its part in the deal due to millions of dollars remaining frozen in a Macau bank account.
The US insisted last month that plans were in place to recover the funds amid fears that the delay may affect the February 13th deal which hopes to see North Korea shut down its nuclear facilities and allow UN inspectors to investigate the sites.
It is now up to Pyongyang to make the next move, the US said on Saturday.
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