North Korea says it is restarting all operations at a shuttered nuclear reactor, adding to already heightened tensions on the Korean peninsula.

The official Korean Central News Agency quoted officials as saying Pyongyang would "readjust and restart" the facilities at the Yongbyon complex both to build more bombs and address the country's electricity shortage.

It said this will include a uranium enrichment plant and a five megawatt reactor that can generate spent fuel rods laced with weapons-grade plutonium. The reactor was shut down in 2007 in exchange for economic aid.

The move follows weeks of threats by Pyongyang, which is angry at tough United Nations sanctions passed in response to its third nuclear test and latest satellite launch.

South Korea's foreign ministry quickly condemned the move, calling it "highly regrettable" and urging Pyongyang to abide by its international agreements.

North Korea agreed to mothball the plutonium-based reactor and destroy its cooling tower as part a 2007 aid-for-disarmament deal at the now-stalled six-party talks.

The North Korean spokesperson said work would begin immediately on refurbishing the reactor. Experts say it would take at least a year of restarted operations to generate enough plutonium to make one nuclear bomb.

Pyongyang is currently believed to have enough plutonium to make up to eight bombs. Many believe a separate uranium enrichment facility at Yongbyon unveiled in 2010 is giving it a second way to make nuclear weapons, although Pyongyang has insisted it is only for generating electricity.

North Korea's first two nuclear tests, in 2006 and 2009, were carried out using plutonium. It is not yet known whether its latest test in February used plutonium or uranium.