Russia is currently rebuilding its armed forces

Mr Putin said the move to resume the flights permanently after a 15-year suspension was in response to security threats posed by other military powers.

He said 14 bombers had taken off from Russian airfields early on Friday.

The move came a week after Russian bombers flew within a few hundred miles of the US Pacific island of Guam.

A few days ago Moscow said its strategic bombers had begun exercises over the North Pole.

Flexing muscles

"We have decided to restore flights by Russian strategic aviation on a permanent basis," Mr Putin told reporters at joint military exercises with China and four Central Asian states in Russia's Ural mountains.

"In 1992, Russia unilaterally ended flights by its strategic aircraft to distant military patrol areas. Unfortunately, our example was not followed by everyone," Mr Putin said, in an apparent reference to the US.

In pictures: War games New era of discord

"Flights by other countries' strategic aircraft continue and this creates certain problems for ensuring the security of the Russian Federation," he said.

In Washington, state department spokesman Sean McCormack played down the significance of Russia's move, saying: "We certainly are not in the kind of posture we were with what used to be the Soviet Union."

"If Russia feels as though they want to take some of these old aircraft out of mothballs and get them flying again, that's their decision," he told reporters.

One of the reasons Russia halted its flights 15 years ago was that it could no longer afford the fuel.

Today Moscow's coffers are stuffed full of oil money, says the BBC's Rupert Wingfield-Hayes in Moscow, and the Kremlin is determined to show it is still a military power to reckon with.

'Shadowed by Nato'

Russian media reported earlier on Friday that long-range bombers were airborne, and that Nato jets were shadowing them.

Itar-Tass quoted Russian air force spokesman Alexander Drobyshevsky as saying: "At present, several pairs of Tu-160 and Tu-95MS aircraft are in the air over the Atlantic and Pacific oceans, which are accompanied by Nato planes."

Nato said it was aware of the flights but had no comment on whether Nato planes were in attendance.

In last week's incident near Guam, the Russian pilots "exchanged smiles" with US fighter pilots who scrambled to track them, a Russian general said.

The US military confirmed the presence of the Russian bombers near Guam, home to a large US base.

Last month two Tupolev 95 aircraft - dubbed "bears" according to their Nato code-name - strayed south from their normal patrol pattern off the Norwegian coast and headed towards Scotland. Two RAF Tornado fighters were sent up to meet them.

Russian bombers have also recently flown close to US airspace over the Arctic Ocean near Alaska.