At least 38 people were killed and more than 60 injured in two suicide bomb attacks on the Moscow Metro during the morning rush hour, officials say.

Moscow's Metro is one of the most-used underground railways in the world, carrying about 5.5 million passengers a day.

The system was partially disrupted following the attacks, but damage to the stations was minimal and both had reportedly reopened by the evening rush hour.

President Medvedev asked officials to increase security on the public transport system nationwide.

"What was being done needs to be substantially strengthened," he said. "Look at this problem on the scale of the state, not only as it applies to a particular type of transport and a particular city."

Female suicide bombers are believed to have carried out the attacks on trains that had stopped at two stations in the heart of the Russian capital.

No group has yet claimed responsibility for being behind the attacks.

But Russian security services believe the bombers are linked to militant groups in the North Caucasus region.

Past suicide bombings in the capital have been carried out by or blamed on Islamist rebels fighting for independence from Russia in Chechnya.

Prime Minister Vladimir Putin cut short a visit to the Siberian city of Krasnoyarsk and said a crime that was "terrible in its consequences and heinous in its manner" had been committed.

"I am confident that law enforcement bodies will spare no effort to track down and punish the criminals. Terrorists will be destroyed," he added.

President Dmitry Medvedev echoed his words after laying a wreath at the site of one of the attacks, saying: "They are animals. I have no doubt that we will find and destroy them all."

The first explosion tore through the second carriage of a train at 0756 (0356GMT), as it stood at central Lubyanka station waiting for morning rush hour commuters to board.

The station, on both the busy Sokolnicheskaya and Tagansko-Krasnopresnenskaya lines, lies beneath the headquarters of the Federal Security Service (FSB).

"I was moving up on the escalator when I heard a loud bang, a blast. A door near the passageway arched, was ripped out and a cloud of dust came down on the escalator," an eyewitness named Alexei told Rossiya 24 TV channel.

"People started running, panicking, falling on each other," he said.

The second blast at Park Kultury, which is six stops away from Lubyanka on the Sokolnicheskaya line, came at 0838 (0438 GMT). It struck at the back of the train as people were getting on board.

"I was in the middle of the train when somewhere in the first or second carriage there was a loud blast. I felt the vibrations reverberate through my body," one passenger told the RIA news agency.

"People were yelling like hell," he said. "There was a lot of smoke and within about two minutes everything was covered in smoke."

The security services said the bomb that went off at Lubyanka station had an equivalent force of up to 4kg of TNT, while the bomb at Park Kultury was equivalent to 1.5-2kg of TNT.

The devices - believed to have been made with the powerful explosive, hexogen, which is more commonly known as RDX - were filled with chipped iron rods and screws for shrapnel.

"The whole city is a mess, people are calling each other, the operators can't cope with such a huge number of calls at a time," said Olga, a BBC News website reader in Moscow. "Those who witnessed the tragedy can't get over the shock."

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