MOSCOW Russian intelligence services are pursuing a network of Islamic State (ISIS) militants operating in Russia as the country faces an increase in terrorist activity, in response to Moscow's recent air campaign in Syria.

Kommersant has learned that a group of terror suspects, said to be trained in Syria, were detained after a raid this week on an apartment in western Moscow where officers seized and deactivated a homemade explosive, with a yield of about five kilograms of TNT equivalent.

Three young Chechen men have since appeared in court. Investigators say that one of those arrested, Aslan Baysultanov, was the organizer and that the group had planned to carry out an attack on a transport facility in Moscow, most likely the metro.

Before travelling by train and then bus to the Russian capital, Baysultanov is believed to have arrived in Chechnya from Syria to carry out terrorist attacks, according to sources at the nation's central intelligence organization, the Federal Security of the Russian Federation (FSB).

Guns, grenades, and detonators were seized. The FSB also claims Baysultanov took part in an ISIS training camp. The suspects could face 20 years prison if convicted of the charges of preparing a terrorist attack and production and distribution of explosives. The three have been remanded in custody until Dec. 12.

The search is on for several other suspects, including Elbrus Batirov, from the Kabardino-Balkar Republic in the North Caucasus. Batirov, who was once a fighter in his native Chegemsky distric near the border with Georgia, later led a local gang before escaping to Turkey.

The FSB has been targeting homegrown threats for a while. Its large-scale operation started with the arrest of North Ossetian suspect Rashid Yevloyev who had been wanted since 2014 for carrying out terrorism training.

Yevloyev's parents said he was about to receive an Islamic education in Turkey before he secretly crossed the Turkish-Syrian border and went to a camp in Aleppo, joining others from the South Caucasus for combat training. Yevloyev's lawyers have told Kommersant that their client has kept mum about his activities.

Meanwhile, the FSB and Russia's Internal Affairs Ministry responsible for Chechnya have been instructed to be on the lookout for other potential militants who have also arrived in the Caucasus from Syria.

The warning comes as ISIS called for jihad to be waged on Russia as Moscow intensifies its air campaign against Islamic militants in Syria.

In an online statement, ISIS spokesman Abu Mohammad al-Adnani appealed to jihadists in the Caucasus, saying that "if the Russian army kills the people of Syria, then kill their people."

Russia said that its air force has hit 86 terrorist targets in Syria within 24 hours this week — the highest one-day tally since it launched its bombing campaign on Sept. 30.