At least 35 people have been killed and more than 150 injured in a blast at Moscow's busiest airport, health officials said.
The explosion at Domodedovo airport, in the southeast of the Russian capital, occured on Monday afternoon inside the airport's international arrivals hall near the baggage area.
"Today at 4.32 pm (13:32 GMT) an explosion went off in the international arrivals hall of Domodedovo airport," the Russian investigative committee said in a statement.
Dmitry Medvedev, the Russian president, said that the blast, according to preliminary information, "was an act of terror" and that those behind it would be tracked down and punished.
He also called for a new security regime to be introduced in all airport and transport hubs across the country in the wake of the attack.
World leaders have condemned the attack, with Barack Obama, the US president, calling it "an outrageous act of terrorism against the Russian people".
Scenes of carnage
Eyewitnesses told Russian radio of a scene of carnage after the blast ripped through the baggage claims section of the airport.
"Burned people are running about ... they are carrying pieces of flesh on stretchers," a man called Andrei, who was standing near the information stand at the airport, told City FM radio.
"You can't tell the living from the dead. I was meeting someone. We are not protected in this country," another witness, Alexei, told the station.
A traveller named Viktor told the Russkaya Sluzhba Novostei radio station he heard a loud bang outside the airport.
"There was an explosion, a bang. Then I saw a policeman covered in fragments of flesh and all bloody. He was shouting 'I've survived! I've survived!'''
Neave Barker, Al Jazeera's correspondent in Moscow, said that Domodedovo airport is the busiest of the Moscow's three commercial airports, serving 22 million people last year.
"It's usually very, very busy in there ... This is a deadly target for this attack," he said.
"Many officials and journalists would have been flying out of Moscow to the Davos forum around this time. It's not clear whether this was timed to have the biggest international impact."
The Kremlin said that Medvedev was delaying his own trip to Davos as a result of the attack.
Moscow police have stepped up security across the city following the blast.
But Russian media reports suggested that security services had received warning of a possible attack ahead of Monday's blast.
"The special services had received information that an act of terror would be carried out at one of the Moscow airports," the RIA Novosti news agency reported an unnamed security source as saying.
"Agents were seeking three suspects but they managed to access the territory of the airport, witness the explosion which their accomplice carried out and then leave the airport," the source said.
Separately, an airport security source told the Lifenews.ru website: "A tip-off with a warning that something was being prepared appeared one week before the explosion."
Russia's Interfax news agency later reported that investigators had found a head of "Arab appearance" that they presumed to have belonged to a suicide bomber.
The Russian capital is no stranger to attacks. In March last year 40 people were killed and 100 wounded when two female suicide bombers attacked the Moscow metro system.
That and previous attacks have been blamed on fighters from the Northern Caucasus, where separatists are fighting for an autonomous state.
Some commentators have already pointed towards separatists in the Caucasus as being behind Monday's attack.
"There is a very, very strong suspicion that insurgent fighters operating in the likes of Chechnya, Ingushetia and Dagestan are responsible for carrying out this latest bombing," Al Jazeera's correspondent said.
Source: Al Jazeera and agencies