MOSCOW (AFP/REUTERS) - Russia's security services it has foiled a "terrorist attack" on Moscow's public transport system by a group of militants, some of whom were trained by ISIS.
A statement by the federal security service (FSB) said on Monday they had detained a group of people in Moscow and seized bomb-making materials they were planning to use in an attack on the capital's busy transport system.
"During questioning of two of the detained men, it became known that they were planning a terrorist attack on the Moscow public transit system," said the statement which was carried by Russian news agencies.
Russian security forces had on Sunday arrested at least three people in a raid on an apartment block in central Moscow where security forces also found an explosive device, with dozens of people evacuated from the area.
The raid came after the security services identified a Moscow address "where six to 11 people periodically lived, some of who went through combat training in Islamic State camps in Syria", it said.
"Some of these people underwent training in Islamic State in Iraq and Syria (ISIS) camps in Syria," Interfax news agency quoted the FSB as saying. "Questioning of two of the detainees revealed that that were planning to carry out a terrorist attack on public transport in Moscow."
The FSB said the suspects arrived back in Russia long before Moscow launched a bombing campaign in Syria late last month.
It was not immediately clear how many people had been arrested in total.
But their arrests will be seen by the Kremlin as supporting its argument that the intervention is necessary to keep home-grown extremist militants at bay.
"Instead of waiting for them to return home, we should help President Bashar al-Assad fight them there," Putin told US television before meeting US President Barack Obama in September.
Some Russians have argued, however, that Putin's actions could make Russia a target of revenge attacks.
"The threat of terrorist activity has risen sharply in connection with the beginning of a mass bombing campaign in Syria," said Gennady Gudkov, an opposition activist and former deputy head of the Russian Parliamentary Security Committee.
"Moreover, it has increased specifically in Moscow," he told the radio station Echo Moskvy.
Footage of Sunday's raid shown on Rossiya state TV channel showed police officers carrying sacks of "evidence" out of the building, including a washing machine that "could have been used as a hiding place".
Officers found a device consisting of a plastic bucket with ammonia and aluminium powder, complete with a phone-activated detonator, the report said.
Moscow claims it is targeting ISIS and other "terrorist" groups, but the US and its ally say Russia is bombing more moderate rebels battling Assad's forces.
Russia - which has battled its own extremist insurgency in the volatile North Caucasus - says that some 2,000 of its citizens are fighting in the ranks of IS.
The Russian capital has a grim history of militant attacks going back more than a decade. Most recently, two suicide bombers killed 40 people on the Moscow metro in 2010, and in 2011, more than 30 people were killed in a suicide bombing at Moscow's Domodedovo airport.
The Kremlin said on Monday that the heads of the former Soviet states holding a summit in Kazakhstan on Friday would endorse the creation of a joint task force to defend their external borders.