By Stephen Dalziel BBC Russian Affairs analyst in Moscow

They aim to check the effectiveness of Russia's nuclear shield.

Russian media has suggested Mr Putin wants to use the event to show his faith in the armed forces ahead of next month's presidential election.

But the launch of two submarine-based ballistic missiles was reported to have failed on Tuesday.

Mr Putin was embarrassed by reports of failed missile launches

Mr Putin's preparation for the presidential election is looking increasingly like his preparation four years ago.

There has been no official election campaign; massive coverage of his movements in the Russian media; and Mr Putin being seen as someone with a close understanding of military matters.

Shortly before the last election, he co-piloted a jet fighter into Chechnya.

On Tuesday, he went on board a nuclear submarine, the Arkhangelsk, to observe the biggest military exercises to take place during his presidency.

There is an added symbolism, in that the submarine is in the Barents Sea.

It was there in August 2000 that the submarine Kursk sank, killing all 118 men on board.

The accident itself, and the slow way in which Mr Putin reacted, made it the blackest spot on his presidency.

If he thought that this was a chance to show the strength of the armed forces... he has had a notable setback

Perhaps Mr Putin is hoping to exorcise some of the ghosts of the Kursk disaster now.

But if he thought that this was a chance to show the strength of the armed forces, under his watchful eye as commander-in-chief, he has had a notable setback.

One of the highlights of the manoeuvres was supposed to be the launch of two ballistic missiles from a submarine in the Barents Sea, to strike a target on the Kamchatka Peninsula, over 5,000 km away.

Each launch failed, apparently because of a satellite error, according to information issued by the Northern Fleet.

Denials

But three hours later, Naval Commander-in-Chief Vladimir Kuroyedov said that this was not the case, as the launches were supposed only to be "virtual" - in other words, all procedures would be followed up to, but not including, an actual missile launch.

Three facts stretch the credibility of this claim. Firstly, when the Kursk sank, the naval high command put out information which was soon shown to be false.

Secondly, the Northern Fleet command already admitted to the misfiring of the missiles.

And thirdly, when the exercises began last week, it was announced that one of the highlights would be the launch of missiles from the Barents Sea to strike targets on the Kamchatka Peninsula.

If this was not to be done with actual missiles, what was the point in Mr Putin himself saying that the exercise was designed to test the efficiency of Russia's nuclear shield?