Two previously unidentified underground complexes have been identified at North Korea's mainstay nuclear compound in Yongbyon, north of Pyongyang, a US website monitoring the communist state has said.

In an expert analysis of satellite imagery by Frank V. Pabian, 38 North on Thursday revealed the underground complexes, located east and southeast of Yongbyon across the Kuryong River, saying their purposes remain unknown.

This image shows North Korea's main nuclear complex in Yongbyon, north of Pyongyang. (Yonhap)

The findings are based on the presence of tunnel entrances and visible spoil piles, which 38 North said have become far less obvious with time.

"While it is impossible to remotely discern their purposes, their location within Yongbyon's security perimeter and subsequent camouflaging qualify them as subjects of interest for future inspection teams," the website said in the analysis.

One tunnel complex lies directly across the river from the 5 megawatt nuclear reactor, as indicated by the presence of excavated spoil piles, the website said.

The other tunnel was excavated under a hillside along the Kuryong River, northeast of the abandoned 50 megawatt reactor area, it added.

The analysis came as Washington and Pyongyang are preparing to resume their working-level nuclear negotiations as early as this month. Their talks may touch on the dismantlement of the Yongbyon facilities in return for US concessions.

(Yonhap)