Who is to blame? What Russia can do Timeline: The blasts which shook Russia

Seventeen people were killed and more than 100 people injured when a lorry bomb tore through an apartment block in Volgodonsk near Russia's volatile northern Caucasus region.

It was the fifth such blast in just over a fortnight and the third to target a block of flats in a week.

Police are reported as saying that an explosion at an apartment block in St Petersburg on Thursday night, which killed two people, was also caused by a bomb. First reports said a gas leak had been responsible.

The Itar-Tass news agency says the blast was caused by an explosive device equivalent to 4-5kg of TNT, and a subsequent blaze was made worse by several containers of petrol kept in one of the flats.

Moscow has blamed Muslim militants from the breakaway republic of Chechnya for the bomb attacks.

The BBC's Andrew Harding: "This appears to be a random attack"

As rescuers sifted through the rubble of the block, officials announced that police had discovered 3.5 tonnes of explosives in a southern Moscow suburb.

The explosives were hidden among sacks of sugar from a plant in southern Russia. Six timing devices were also reportedly found.

'We'll stamp out vermin'

Russian Prime Minister Vladimir Putin vowed to crush the wave of violence sweeping the country following the latest blast.

"We should not whimper and whine, we all need to act decisively, determinedly and energetically," he said. "We must stamp out this vermin."

President Boris Yeltsin, who met Mr Putin to discuss the explosion, said the Chechen border should be sealed so that ''not a single transportation artery'' remained open.

"We have the strength of will and the means to wipe out terrorism,'' he added.

Alexander Konovalov of the Institute of Strategic Assessment: "The Russian state is in danger"

In a message of condolence to the regional governor he branded the attack "a barbaric act of terrorism".

"Again there is an attempt to intimidate the people of Russia, to sow terror and panic. But the terrorists will not get their way. They will not break us,'' he added.

Security checks

The bomb attacks have prompted a major security operation in Moscow and other large Russian cities with police sweeping apartment blocks for explosives.

Moscow correspondent Robert Parsons: "Rescue efforts being hampered by fire"

Reports said the Volgodonsk building had cleared a security check shortly before the blast.

''We checked both the basements and attics,'' police spokesman Dmitry Nevzorov added. "We locked these parts of the premises, which is perhaps why the terrorists did not place the explosives in the building but outside in a truck.''

Police put the force of the blast at 300-350 kilogrammes (660-770 pounds) of TNT, but explosives experts have not yet determined what chemicals were used.

The explosion, at about 0600 local time (0200GMT), brought down the front of the block which housed around 100 apartments.

It also damaged a nearby police station and about 20 other buildings. Several floors were engulfed in flames.

Moscow bombs

The blast follows two devastating bomb attacks in Moscow, one on Monday which killed 118 people and the other seven days ago, which killed 94.

Other recent targets have included a military housing complex in Dagestan and a shopping mall near the Kremlin.

On Wednesday, Mr Putin said the people who carried out the suspected bombings in Moscow were hiding in Chechnya.

He said Russia would ask the Chechen Government to "hand over the criminals".

The government has not provided any evidence of the militants' involvement in the blasts.