Russia said on Saturday it had arrested 11 people including four suspected gunmen in connection with a shooting rampage that killed 143 people in a concert hall near Moscow, the deadliest attack in Russia for 20 years.

Militant Islamist group Islamic State claimed responsibility for Friday's attack but there were indications that Russia was pursuing a Ukrainian link, despite emphatic denials from Ukrainian officials that Kyiv had anything to do with it.

The FSB security service said "all four terrorists" had been arrested while heading to the Ukrainian border, and that they had contacts in Ukraine. It said they were being transferred to Moscow.

"Now we know in which country these bloody bastards planned to hide from pursuit - Ukraine," Foreign Ministry spokeswoman Maria Zakharova said on Telegram.

A senior Russian lawmaker, Andrei Kartapolov, said that if Ukraine was involved, then Russia must deliver a "worthy, clear and concrete" reply on the battlefield.

Ukrainian military intelligence spokesman Andriy Yusov said: "Ukraine was of course not involved in this terror attack. Ukraine is defending its sovereignty from Russian invaders, liberating its own territory and is fighting with the occupiers’ army and military targets, not civilians."

He said the FSB version that the suspects were arrested en route to Ukraine was "of course another lie from the Russian special services".

Russian state TV editor Margarita Simonyan published a video of one of the suspects, a young bearded man, being interrogated by the side of a road. She said the death toll had climbed to 143 but did not give the source of her information.

Russia's Investigative Committee earlier said at least 115 had died in the attack, in which camouflage-clad gunmen opened fire with automatic weapons at concertgoers in the Crocus City Hall near the capital.

It said some died from gunshot wounds and others in a huge fire that broke out in the complex. Reports said the gunmen had lit the blaze using petrol from canisters they carried in rucksacks.

People fled in panic. Baza, a news outlet with good contacts in Russian security and law enforcement, said 28 bodies were found in a toilet and 14 on a staircase. "Many mothers were found embracing their children," it said.

Russian lawmaker Alexander Khinshtein said the attackers had fled in a Renault vehicle that was spotted by police in Bryansk region, about 340 km (210 miles) southwest of Moscow on Friday night and disobeyed instructions to stop.

He said two were arrested after a car chase and two others fled into a forest. From the FSB statement, it appeared they too were later detained.

Khinshtein said a pistol, a magazine for an assault rifle, and passports from Tajikistan were found in the car. Tajikistan is a mainly Muslim Central Asian state that used to be part of the Soviet Union.

SUSPECT INTERROGATED

The suspect in the interrogation video was shown replying in heavily accented Russian to a series of barked questions. He said he had flown from Turkey on March 4 and had received instructions from unknown people via Telegram to carry out the attack in exchange for money.

The man was trembling throughout the questioning. He was initially shown lying on his stomach with his hands bound behind his back, his chin resting on the boot of a figure in camouflage uniform. Later he was hauled up onto his knees.

Another man with cuts and bruises to his face was shown being questioned via an interpreter while sitting on a bench with bound hands and feet.

Russia has yet to publicly present evidence of any Ukrainian link to the attack.

President Vladimir Putin, re-elected last Sunday for another six-year term, has repeatedly told Russians that various powers - including countries in the West - are seeking to sow chaos inside Russia.

The Kremlin said Putin had held conversations with the leaders of Belarus and Uzbekistan in which all sides affirmed their willingness to work together to fight terrorism.

GUNFIRE AND SCREAMS

Verified video of Friday's attack showed people taking their seats in the concert hall, then rushing for the exits as repeated gunfire echoed above screams.

Long lines formed in Moscow on Saturday for people to donate blood. Health officials said more than 120 people were wounded.

Islamic State, the militant group that once sought control over swathes of Iraq and Syria, claimed responsibility for the attack, the group's Amaq agency said on Telegram.

Islamic State said its fighters attacked on the outskirts of Moscow, "killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely". The statement gave no further detail.

The United States has intelligence confirming Islamic State's claim of responsibility for the shooting, a U.S. official said. The official, speaking on condition of anonymity said Washington had warned Moscow "appropriately" in recent weeks of the possibility of an attack.

Friday's attack, about 20 km (12 miles) from the Kremlin, happened two weeks after the U.S. embassy in Russia warned that "extremists" had imminent plans for an attack in Moscow.

Hours before the embassy warning, the FSB said it had foiled an attack on a Moscow synagogue by Islamic State's affiliate in Afghanistan, known as ISIS-Khorasan or ISIS-K, which seeks a caliphate across Afghanistan, Pakistan, Turkmenistan, Tajikistan, Uzbekistan and Iran.

Putin changed the course of the Syrian civil war by intervening in 2015, supporting President Bashar al-Assad against the opposition and Islamic State.

"ISIS-K has been fixated on Russia for the past two years, frequently criticizing Putin in its propaganda," said Colin Clarke of the Soufan Center.

The broader Islamic State group has claimed deadly attacks across the Middle East, Afghanistan, Pakistan, Iran, Europe, the Philippines, and Sri Lanka.

WORLD REACTION

Russian Foreign Ministry spokeswoman Zakharova said it was a "bloody terrorist attack" that the world should condemn.

The United States, European and Arab powers and many former Soviet republics expressed shock and sent their condolences. The United Nations Security Council condemned what it called a "heinous and cowardly terrorist attack".

Russia tightened security at airports, transport hubs and across the capital - a vast urban area of over 21 million people - and big public events were cancelled across the country.