The Democratic People's Republic of Korea put its missile units on standby on Friday to attack US military bases in the Republic of Korea and the Pacific region, after the United States flew two nuclear-capable stealth bombers over the Korean Peninsula in a rare show of force.

DPRK leader Kim Jong-un signed off on the order at a midnight meeting of top generals and "judged the time has come to settle accounts with the US imperialists in view of the prevailing situation", the KCNA news agency said.

KCNA said the DPRK and the US could only settle their differences by "physical means". The DPRK has an arsenal of Soviet-era short-range Scud missiles that can hit the ROK, but its longer-range Nodong and Musudan missiles, which could in theory hit US Pacific bases, are untested.

China repeated its calls for restraint on the Korean Peninsula at a regular Foreign Ministry briefing.

"We hope that relevant parties will work together in pushing for a turnaround of the tense situation," ministry spokesman Hong Lei said.

Tension has been high since the DPRK conducted a third nuclear weapons test in February in breach of UN sanctions and despite warnings from China not to do so.

Russia's foreign minister implicitly criticized the US bomber flights.

"We are concerned that alongside the adequate, collective reaction of the UN Security Council, unilateral action is being taken around the DPRK that is increasing military activity," Russian Foreign Minister Sergei Lavrov said.

"The situation could simply get out of control. It is slipping toward the spiral of a vicious cycle," Lavrov said in Moscow when asked about the situation.

He called for efforts to get the stalled Six-Party Talks going again. The talks have involved the DPRK, the ROK, the US, Russia, China and Japan.

Threats

On Thursday, the US flew two radar-evading B-2 Spirit bombers on practice runs over the ROK. They flew from the US and back in what appeared to be the first exercise of its kind, designed to show America's ability to conduct long-range, precision strikes "quickly and at will", the US military said.

The news of Kim's response was unusually swift.

"He finally signed the plan on technical preparations of strategic rockets of the Korean People's Army, ordering them to be on standby for fire so that they may anytime strike the US mainland, its military bases in the operational theatres in the Pacific, including Hawaii and Guam, and those in the ROK," KCNA said.

Yonhap News Agency reported there had been additional troop and vehicle movements at the DPRK's mid- and long-range missile sites, indicating they may be ready to fire.

It was impossible to verify the report, which did not specify a time frame. The ROK's Defense Ministry said it was watching shorter-range Scud missile sites closely as well as Nodong and Musudan missile batteries.

The DPRK has launched a daily barrage of threats since early this month when the US and the ROK began regular military drills.

The ROK and the US have said the drills are purely defensive and that no incident has taken place in the decades they have been conducted in various forms.

The DPRK has put its military on highest readiness to fight what it says are hostile forces conducting war drills. Its leader has previously given "final orders" for its military to wage war with the ROK.

The DPRK has canceled an armistice agreement with the US and cut all communications hotlines with US forces, the United Nations and the ROK.

The DPRK has "to understand that what they're doing is very dangerous", US Defense Secretary Chuck Hagel said on Thursday.

"We must make clear that these provocations by (the DPRK) are taken by us very seriously and we'll respond to that."

Victor Cha, a DPRK expert at the Center for Strategic and International Studies, said the drill fitted within the context of ramped-up efforts by the Pentagon to deter the DPRK from acting upon any of its threats.

Asked whether he thought the latest moves could further aggravate tension, Cha, a former White House official, said: "I don't think the situation can get any more aggravated than it already is."