Russian authorities say they have detained four people they claim are members of the extremist group Islamic State (IS) who were plotting an attack on Moscow's transportation network.

In a statement on May 25, the Federal Security Service (FSB) said the detainees, who included citizens of Russia and of unspecified Central Asian countries, were receiving instructions from Syria.

It said that officers who searched the suspects' homes found a lab to make explosives, a handmade explosive device that was ready for use, components of another explosive device, an automatic firearm, ammunition, grenades, and extremist literature.

The FSB also said that the suspects planned to travel to Syria after carrying the attack. It gave no details about the alleged target or time frame.

There was no way to immediately verify the FSB statement.

It came weeks after 16 people were killed on a subway train in St. Petersburg that authorities say was carried out by a 22-year-old ethnic Uzbek Russian citizen born in Kyrgyzstan.

Ten people with roots in Central Asia have been arrested in St. Petersburg and Moscow in connection with the April 3 subway bombing.