The analysts cited what appears to be a small vehicle or trailer right outside the North Portal of the underground facility where the North conducted five previous nuclear tests.

"In the courtyards of the Main Administrative Area, are approximately 11 probable tarp-covered pallets of equipment or supplies, a formation of personnel, and several individuals walking about," they added.

They described the North as "primed and ready" for another nuclear test.

But South Korean military officials said there have been "no unusual" signs detected in connection with reports of the North's possible strategic provocations.

"There has been no unusual activity so far," Army Col. Roh Jae-cheon, a spokesman for the Joint Chiefs of Staff, told reporters.

Another defense official here did not rule out the possibility that the unpredictable North will carry out a nuclear test without special indications spotted, given its past provocation.

"It's routine for the South Korean military to strengthen its alert level" before and after the North's celebration of the 105th birth anniversary of Kim Il-sung, the communist nation's late founding father and grandfather of current leader Kim Jong-un, he added.

Hundreds of foreign journalists are visiting Pyongyang to cover the occasion.

They were reportedly told by North Korean officials to prepare for an unspecified "big and important event." (Yonhap)