SEOUL (AFP) - South Korea's military said on Tuesday it had formed a crisis management task force to monitor North Korea's nuclear site ahead of an expected atomic test, which Seoul said could be imminent.

"Our assessment is that North Korea has almost completed preparations for a nuclear test," defence ministry spokesman Kim Min Seok told reporters, reaffirming Seoul's belief that the test could come any time.

The task force has been set up by the South's Joint Chiefs of Staff and is cross-referencing data and intelligence being collected by state institutions and independent experts, Mr Kim said.

The state-run Korea Institute of Geoscience and Mineral Resources has set up a situation room to receive and process seismic information that might indicate when a test is conducted, he added.

The institute had detected an "artificial earthquake" about 40 seconds after the North's second nuclear test in 2009.

The North has threatened to detonate a nuclear device as a riposte to expanded UN sanctions imposed after Pyongyang's rocket launch in December.

In a meeting Sunday, North Korean leader Kim Jong UN ordered top security officials to take "substantial and high-profile important state measures" - fuelling speculation of an imminent test.