The US State Department on Monday urged North Korea to refrain from escalating tensions after indications emerged of the communist nation resuming operations at its plutonium-producing nuclear reactor.

The North Korea-monitoring website 38 North has reported that recent satellite imagery showed the North restarting the 5-megawatt reactor at its Yongbyon nuclear complex after completing reprocessing spent fuel unloaded from the reactor.

The State Department declined to confirm the report, but urged restraint.

"We are not going to comment on matters of intelligence. We continue to call on North Korea to refrain from actions and rhetoric that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments," Katina Adams, a State Department spokeswoman, told Yonhap News Agency.

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods that, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its first three nuclear tests, in 2006, 2009 and 2013. The North conducted its fourth and fifth nuclear tests in January and September last year.

The restarting of the reactor has added to the already high tensions after North Korean leader Kim Jong-un said in his New Year's Day address that the country has entered the final stage of preparations to test-fire an intercontinental ballistic missile apparently capable of striking the US. (Yonhap)