RUSSIA has warned heightened military activity near North Korea is slipping into a "vicious cycle" that could get out of control, implicitly criticising US bomber flights that led to a ramping up of action by Pyongyang.

Russian Foreign Minister Sergei Lavrov suggested North Korea should also cool down, calling on all sides not to "flex their military muscle" .

Yesterday North Korea put missile units on standby to attack US bases in South Korea and the Pacific, after the United States flew two nuclear-capable stealth bombers over the Korean peninsula in a rare show of force. North Korean leader Kim Jong-un signed the order at a midnight meeting of top generals and the official KCNA news agency said he "judged the time has come to settle accounts with the US imperialists."

The agency said North Korea and the United States could only settle their differences by "physical means".

The North has an arsenal of short-range Scud missiles that can hit South Korea, and longer-range but untested Nodong and Musudan missiles that could in theory hit US Pacific bases.

Tension has been high since North Korea conducted a third nuclear weapons test in February, in breach of UN sanctions and despite warnings from China, its only major ally, not to do so.

Mr Lavrov said: "We are concerned that alongside the adequate, collective reaction of the UN Security Council, unilateral action is being taken around North Korea that is increasing military activity."

He added: "The situation could simply get out of control: it is slipping toward the spiral of a vicious cycle."

He called for efforts to get stalled talks on North Korea going. The talks have involved the two Koreas, the US, Russia, China and Japan.

China repeated its calls for restraint on the Korean peninsula and made no criticism of the US flights.

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

The US sent two radar-evading B-2 Spirit bombers on practise runs over South Korea on Thursday, responding to North Korean threats. They flew from the US 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.

South Korea reported extra troop and vehicle movements at the North's mid- and long-range missile sites, indicating they may be ready to fire.

The North has launched a daily barrage of threats since early this month, when the US and the South began regular military exercises.

Few believe North Korea will risk starting a full-scale war: despite rhetoric from the North, it has kept open a joint economic zone with the South that generates $2 billion a year in trade

But US Defence Secretary Chuck Hagel said on Thursday : "The North Koreans have to understand that what they're doing is very dangerous. It has ratcheted up the danger and we have to understand that reality."

Mr Hagel is bolstering missile defences over the growing North Korea threat and said all of the actions by the country had to be taken seriously.