Russian security forces said on Monday that militants trained by the ISIS terror group had planned to strike Moscow's public transport system, but police forces successfully prevented the plot.

Authorities in the Russian capital arrested several terror suspects, and intelligence officials claimed that some of those in custody had been trained by ISIS in Syria, where the group controls large swaths of territory, the Associated Press reported.

News of the planned attack comes on the heels of Moscow's increased involvement in the Syrian civil war, where Russia has launched airstrikes against targets threatening the rule of dictator Bashar al-Assad. President Vladimir Putin acknowledged on Sunday the move increased the likelihood of terror attacks, but insisted his government's policy was necessary.

"If we just stood by and let Syria get gobbled up, thousands of people running around there now with Kalashnikovs would end up on our territory," Putin said in an interview with Rossiya state television broadcast. "And so we are helping President Assad fight this threat before it reaches our borders."

Russia's Federal Security Service (FSB), the successor to the Soviet KGB, said in a statement that two of the individuals arrested confessed to a planned attack on Moscow's transportation infrastructure, the semi-official RT network noted.

The domestic security agency added that a device with 11 pounds of explosives was found in an apartment rented by the suspects, according to the AP.

"It was determined that at a certain address in Moscow, six to 11 people have occasionally been, some of them had undergone military training in camps of Islamic State terrorists on Syrian territory and arrived in Russia long before the start of Russian military operation in that country," the FSB detailed in a statement.

If convicted, the suspected militants now in custody could face up to 20 years in prison, the Guardian reported. Meanwhile, it was not clear if any members of the terror cell were still at large.

Up to 2,400 Russians citizens have joined ISIS, and authorities worry that they may eventually pose a threat if they return to Russia, the AP reported.