"Literally several days ago … a large terrorist attack was averted at the preparation stage in the Moscow area," Bortnikov told President Dmitry Medvedev in televised comments.

Bortnikov said the suspects had planned to target "crowded facilities and transport infrastructure" and that security officers had confiscated homemade bombs, other weapons and a map with an attack plan.

He said four suspects from the North Caucasus had been detained and that accomplices had been identified.

Insurgent leader Doku Umarov claimed responsibility for a suicide bombing that killed 37 people at Moscow's Domodedovo Airport in January and twin bombings on the Moscow metro that killed 40 in March 2010.

Umarov has said he has readied dozens of potential suicide bombers and has threatened more attacks this year.

Bortnikov, speaking as part of a semi-annual report to Medvedev on the FSB's fight against militants, said 169 "terrorist" crimes had been recorded in Russia this year, 110 of them in Dagestan.

He said 95 law enforcement and security agents had been killed and more than 200 wounded fighting militants this year.

Medvedev said the number showed that "the situation remains highly, highly tense" but praised the FSB for preventing what Bortnikov said were 52 planned attacks in that period.

Separately, Nezavisimaya Gazeta reported that the Air Force began large-scale training maneuvers in the North Caucasus on Monday in a step toward boosting security in the region.

The exercise is the largest of its kind in Kabardino-Balkaria in 15 years and will involve fighter jets and combat helicopters, the report said, citing Air Force spokesman Vladimir Drik.

The war games will continue until Aug. 10, Drik said.

The Winter Olympics will be held in Sochi in the nearby Krasnodar region in 2014.

Insurgent violence has surged in recent months in Kabardino-Balkaria, where militants gunned down three Moscow tourists en route to ski at Mount Elbrus, a popular local resort and the tallest peak in Europe.