Two female suicide bombers killed at least 38 people on packed Moscow metro trains during rush hour on Monday, stirring fears of a broader campaign in Russia's heartland by Islamists from the North Caucasus.

Prime Minister Vladimir Putin, who cemented his power in 1999 by launching a war to crush Chechen separatism, broke off a trip to Siberia, declaring "terrorists will be destroyed."

Witnesses described panic at two central Moscow stations after the blasts, with commuters falling over each other in dense smoke and dust as they tried to escape the worst attack on the Russian capital in six years.

Sixty-four others were injured, many gravely, and officials said the death toll could rise. Russia's top security official said the bombs were filled with bolts and iron rods.

No group immediately claimed responsibility, but Federal Security Service (FSB) chief Alexander Bortnikov said those responsible had links to the North Caucasus, a heavily Muslim region plagued by insurgency whose leaders have threatened to attack cities and energy pipelines elsewhere in Russia.

"A crime that is terrible in its consequences and heinous in its manner has been committed," Putin told emergency officials in a video call.

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

The Kremlin had declared victory in its battle with Chechen separatists who fought two wars with Moscow. But violence has intensified over the past year in the neighbouring republics of Dagestan and Ingushetia, where Islamist militancy overlaps with clan rivalries and criminal rings amid poverty.

The chief of the FSB, the main successor to the Soviet-era KGB, said: "Body parts belonging to two female suicide bombers were found...and according to initial data, these persons are linked to the North Caucasus."

Monday's metro attacks are likely to turn the insurgency in the North Caucasus into a major political issue for Russia's leaders. Critics said Monday's the attacks demonstrated the failure of Kremlin policy in Chechnya, where human rights groups accuse Russian forces of brutal actions.

The first blast tore through the second carriage of a metro train just before 8 a.m. as it stood at the Lubyanka station, close to the headquarters of Russia's main domestic security service, the FSB. It killed at least 23 people.

A second blast, less than 40 minutes later in the second or third carriage of a train waiting at the Park Kultury metro station, opposite Gorky Park, killed 12 more people, emergencies ministry officials said. Another three people died in hospital.

Reuters photographers saw body bags being brought out of both stations. Some of the wounded were airlifted to hospitals in helicopters and central Moscow was brought to a standstill as police closed off major roads.

"It was very scary. I saw a dead body," said Valentin Popov, a 19-year-old student travelling on a train to the Park Kultury station, told Reuters.

"Everyone was screaming. There was a stampede at the doors. I saw one woman holding a child and pleading with people to let her through, but it was impossible."

U.S. President Barack Obama condemned the bombings as did European Union leaders.

"The American people stand united with the people of Russia in opposition to violent extremism and heinous terrorist attacks that demonstrate such disregard for human life, and we condemn these outrageous acts," Obama said.

NO COMPROMISE

The Russian rouble fell sharply on the bombings, but later regained ground, with traders arguing the bombs were unlikely to undermine the strength of the currency.

Russia's benchmark Eurobond due in 2030 was little changed, yielding about 4.99 percent. The rouble-denominated Micex exchange was up 1.1 percent.

"The Russian stock market is more than stable, the rouble is stable," said Anatoly Darakov, head of Russian equity trading at Citi in Moscow. "It's not the first blast in Moscow."

Eye witnesses spoke of panic after the blasts, which ripped through stations just a few kilometres from the Kremlin.

"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," an unidentified man who was on a train at Park Kultury told RIA news agency.

Surveillance camera footage posted on the Internet showed several motionless bodies lying on the floor or slumped against the wall in Lubyanka station lobby and emergency workers crouched over victims, trying to treat them.

The current death toll makes it the worst attack on Moscow since February 2004, when a suicide bombing killed at least 39 people and wounded more than 100 on a metro train.

Chechen separatists were blamed for that attack. Rebel leader Doku Umarov, who is fighting for an Islamic emirate embracing the whole region, vowed last month to take the war to Russian cities.

"Blood will no longer be limited to our (Caucasus) cities and towns. The war is coming to their cities," the Chechen rebel leader said in an interview on the unofficial Islamist website www.kavkazcenter.com.

Jonathan Eyal, of London's Royal United Services Institute, saw a personal challenge to Putin, who remains the chief power in the land.

"This is a direct affront to Vladimir Putin, whose entire rise to power was built on his pledge to crush the enemies of Russia...It's an affront to his muscular image."

The Chechen rebellion began in the 1990s as a largely ethnic nationalist movement, fired by a sense of injustice over the 1940s transportation of Chechens to Central Asia, with enormous loss of life, by dictator Josef Stalin. Largely since the second war, Russian officials say, Islamic militants from outside Russia have joined the campaign, lending it a new intensity.