"It concerns me that the North is doing excavation work at two tunnels (of the site)," the diplomatic source based in Beijing told Yonhap News Agency. "Lots of analysis and evaluation will be needed to determine whether both of them, when completed, will be used to conduct nuclear tests simultaneously or at a short interval and what nuclear materials will be used."

"There are chances that the North will conduct an additional nuclear test, though I cannot say for sure that it will come around the summit between the US and China (in early April) or Kim Il-sung's birthday (on April 15)," he added. "It could be done around April 25 when it celebrates the anniversary of the Korean People's Army."

The source didn't provide any evidence to back up the speculation.

On Friday, a South Korean military source said that North Korea seems to have finished preparing for another nuclear test and that it is capable of conducting a nuclear test "within hours of Kim Jong-un's order."

The North conducted its fourth and fifth nuclear tests in January and September last year, drawing strong condemnation from the international community which was followed by the toughest-ever sanctions aimed at starving its regime of hard currency needed to advance its nuclear weapons capabilities.

The North test-fired four ballistic missiles early this month.

On March 19, Pyongyang also announced that it had tested a new high-thrust rocket engine, raising speculation for near-future provocations. (Yonhap)