MOSCOW - Russian authorities have foiled a plot by militants who were building fertilizer bombs in a suburban apartment and planning to attack a bullet train between Moscow and St. Petersburg, a newspaper reported on Monday.

Citing security sources, Kommersant revealed details of an alleged plot by militants from the North Caucasus that Russia's top security official had described to President Dmitry Medvedev last month as a "large terrorist attack" targeting transport.

The Kremlin is struggling to contain an Islamist insurgency in its mostly Muslim North Caucasus regions that continues to undermine Russia's security and stability in the wake of two wars against Chechen separatists.

The insurgents claimed responsibility for a bombing that killed 26 people on a Moscow-St. Petersburg train in 2009, as well as suicide bombings that killed 37 people at Moscow's busiest airport in January and 40 in the metro in March 2010.

Militant leader Doku Umarov has vowed more bombings in Russia's heartland. A deadly attack would be a blow to Medvedev and Prime Minister Vladimir Putin ahead of a March 2012 vote in which one of them is expected to run for president.

Kommersant said the suspected mastermind, a 22-year-old native of the North Caucasus province of Kabardino-Balkaria, allegedly recruited at least three other men to help carry out the attack on the Sapsan, a high-speed train connecting Russia's largest cities.

A Federal Security Service (FSB) source told the paper the group made bombs out of ammonium nitrate in a rented apartment near the busy railway line and had planned to place one on the tracks some 20 kilometres (12 miles) north of Moscow.

FSB officials contacted by Reuters declined to comment on the report.

In televised comments last month, FSB director Alexander Bortnikov told Medvedev that "a large terrorist attack" targeting "crowded facilities and transport infrastructure" had been averted in the preparation stage near Moscow.

Kommersant said the suspected mastermind, Islam Khamuzhyev, had recruited two men with whom he played soccer and another man he had met at a mosque, who was to act as the getaway driver.

They had identified a "relatively deserted" spot where "it would be safe to lay a bomb at night" and where a web of roads cutting through the surrounding forest would make for an "easy getaway", it said.