The prime minister said security services were investigating whether the Moscow explosion was the result of negligence, criminal carelessness in handling explosive substances or a terrorist act.

'Blood-thirsty enemy'

Traces of explosive substances have been found in the building

"If it was a terrorist act, we are confronted by a cunning, brazen and blood-thirsty enemy," he said.

Moscow Mayor Yuri Luzhkov insisted that it was the work of terrorists.

"It is a complete certainty," he said, after visiting the site.

He said that traces of explosive substances were found at the site and were likely to have been planted in the office of a company called Delko-2, on the first floor of the building.

The BBC's Andrew Harding reports: "Throughout the capital security forces are on high alert"

Rescue operations are still underway at the apartment block, but hopes are fading of finding more survivors. Officials say that 100 people remained buried in the rubble.

Most people would have been asleep at the time of the explosion - around midnight local time (2000 GMT) on Wednesday .

The blast led to the complete collapse of the central part of the block in the south-east of the city. It also damaged 15 nearby buildings.

Russia's security service has confirmed that some sort of explosive device was to blame rather than a gas leak, as initially suspected.

A preliminary investigation suggested an "industrial explosive device" or "a large amount of pyrotechnic devices" were to blame, it said, adding that the explosive power was equivalent to 300-400kg of TNT.

If confirmed as a bomb, it would be the third such attack in Russia in the past 10 days.

A blast in a shopping centre in central Moscow last week killed one person.

And at the weekend, a car-bomb attack killed scores in an apartment block in the southern Dagestan region, where Russian forces are fighting Islamic guerrillas.

An anonymous caller to the Interfax news agency said the Dagestan blast and this latest attack were "our response to the bombing of villages in Chechnya and Dagestan".

But correspondents say the security forces appear to have no real leads as to which group or groups might be behind the explosions.