Russia warned Friday that a flare-up in tensions between North Korea and the United States could spin out of control, urging all sides involved in the standoff to refrain from muscle-flexing.

"Unilateral actions are being taken around North Korea which manifest themselves in an escalation of military activity," Russian Foreign Minister Sergei Lavrov said.

"We can simply see the situation getting out of control, it would spiral down into a vicious circle," he told reporters at a news conference alongside his Ukrainian counterpart.

"We believe it is necessary for all not to build up military muscle and not to use the current situation as an excuse to solve certain geopolitical tasks in the region through military means," he said, calling on all sides to create conditions for the resumption of talks.

North Korean leader Kim Jong-Un earlier Friday ordered missile units to prepare to strike the US mainland and military bases in the Pacific after US stealth bombers flew over South Korea.

The flights were part of annual drills between the United States and South Korea, which North Korea each year denounces as rehearsals for war.

Pyongyang has been particularly irate this time, angered by UN sanctions imposed after its long-range rocket launch in December and the third nuclear test last month.

But a Russian military source, speaking to the Interfax news agency, dismissed North Korea's promise to strike as an empty threat.

"So far North Korea does not have a delivery vehicle which could reach the United States or its bases in the Pacific Ocean," said the source, who was not named.

"North Koreans are also far from creating a nuclear warhead which could be put on an intercontinental ballistic missile," the source told the news agency.

North Korean state media issued two photos Friday that appeared to show plans for striking the United States.

One picture showed Kim sitting at a desk in what looked like a military operations room. A map is shown in the picture with the unambiguous title: "Strategic Forces' US Mainland Striking Plan."

The Russian source dismissed the pictures as "bogeyman stories".

"I do not believe in the possibility of such strikes," he said, noting that North Korea's threats gave Washington an excuse to increase its military presence in the Pacific.

"Thanks to their rhetoric, the North Koreans in fact play into the hands of Americans, giving them a great excuse to ramp up missile defence capabilities in the Asia-Pacific region," the military source said.