Seoul: North Korea on Tuesday warned the US of a “horrible disaster” and put its troops on alert over a major naval drill involving a US aircraft carrier alongside South Korean and Japanese vessels.

The warning came after Seoul and Washington last week signed a new joint strategy to counter what they called the growing threat of a North Korean nuclear attack after the communist country restarted an ageing plutonium reactor.

That accord also drew a sharp rebuke, with analysts attributing the isolated regime’s recent bellicose rhetoric to its desire to attract the US’ attention and draw it back into dialogue.

The planned naval drill in waters around the Korean peninsula involves South Korea, Japan and the US, which has deployed the nuclear-powered carrier USS George Washington.

North Korean troops had been ordered to “keep themselves fully ready to promptly launch operations any time”, a spokesman for the General Staff of the Korean People’s Army said in a statement on the state news agency.

The North said the situation on the peninsula was “getting strained again” and warned the US that the closer its forces came “the more unpredictable disasters their actions will cause”.

“The US will be wholly accountable for the unexpected horrible disaster to be met by its imperialist aggression forces,” the statement said.

“The US should bear in mind that the Korean people and army are highly alert to promptly and confidently cope with and foil blatant provocations of any hostile forces in the world with its own powerful military muscle.”

The drill, originally scheduled for three days from Tuesday, was postponed for a few days because of an approaching typhoon, Japan’s Chief of the Maritime Staff Katsutoshi Kawano told a Tokyo press conference.

Japan will deploy two escort vessels, the Ise and the Haruyuki, for the drills which will focus on search and rescue exercises, Kawano said, adding Japan will be involved on Thursday and Friday.

He declined to comment on schedules for the US and South Korean navies.

Meanwhile, South Korea’s spy agency confirmed Tuesday that the North has restarted an aging plutonium reactor that could help boost its nuclear weapons programme.

The National Intelligence Service (NIS) said in a report to parliament that the five megawatt reactor at the North’s Yongbyon nuclear complex had resumed operations, according to a joint briefing by ruling and opposition party lawmakers.

The report was presented at a closed intelligence committee session, lawmakers told media.

The spy agency declined to comment on the report.

It followed speculation by the US-Korea Institute at Johns Hopkins University in Baltimore that North Korea had restarted the reactor.

A commercial satellite image taken on September 19 showed the plutonium reactor releasing hot waste water into a river through a new drainpipe, the think tank said last week.

An image from late July had not shown any sign of hot water discharge, however, indicating a recent relaunch.

In reports released last month, the institute and another Washington think tank, the Institute for Science and International Security, observed steam coming from the reactor.

The drainpipe is critical to maintaining a safe temperature at the reactor. North Korea knocked down a cooling tower in 2008 to show its commitment to a US-backed aid-for-disarmament deal.

Pyongyang carried out its third nuclear test in February, sparking international condemnation and raising tensions on the Korean peninsula for months.

Two months later, it boasted that it would restart all facilities at Yongbyon to bolster its atomic arsenal.

Russia has warned that the resumption of Yongbyon could lead to catastrophe. The reactor, a source of great national pride and international anxiety over its role at the heart of North Korea’s nuclear ambitions, was built in 1986 and is outdated.

“Our main concern is linked to a very likely man-made disaster as a consequence. The reactor is in a nightmarish state, it is a design dating back to the 1950s,” a Russian diplomatic source told Interfax news agency last month.

“For the Korean peninsula this could entail terrible consequences, if not a man-made catastrophe.”