North Korea's main nuclear complex showed continued activity at its uranium enrichment facility throughout the winter, a US monitor said Sunday.

In an experts' analysis of satellite imagery from January and February this year, the think tank, 38 North, said the movement of specialized vehicles and equipment near the uranium facility at Yongbyon indicates that the Uranium Enrichment Plant (UEP) has continued operations.

Satellite imagery, however, showed no signs of activity at nuclear reactors, it said.

"In contrast to the reactor area of the complex, the UEP exhibited continual signs of operation throughout the winter months," the website said.

It pointed out that recent imagery shows "uniquely configured, specialized railcars" arriving and departing the UEP complex, which follows the past pattern of arriving two to three times a year at the complex, where they stay for around four weeks, possibly transferring chemical reagents.

The report comes amid a prolonged stalemate in negotiations between Washington and Pyongyang aimed at dismantling the North's nuclear and ballistic missile programs in exchange for sanctions relief. (Yonhap)