Ten militants captured in Russia's North Caucasus last week have been formally arrested in connection with the derailment of a Moscow-St. Petersburg train in November, a respected daily said on Wednesday.

The group of militants was detained during a special operation in the southern Russian republic of Ingushetia on March 2. They were taken to Moscow's Lefortovo prison, the main pretrial detention facility in the capital, Kommersant said.

Six other militants, including the notorious Alexander Tikhomirov, also known as Said Buryatsky, were killed.

Alexander Bortnikov, the head of the Russian Security Service (FSB), said police found a laboratory for making explosive devices and seized more than 1,000 kg of explosives, many small arms and ammunition at the site of the operation in Ingushetia's Nazran district.

A police source told RIA Novosti that the gang was planning another major terrorist attack in the North Caucasus.

Kommersant reported that Moscow's Basmanny District Court on March 3 and 4 authorized the arrest of the militants, who pleaded not guilty, on suspicion of terrorism and illegal weapons dealing.

According to Russian law, an indictment against them must be filed within 30 days of their arrest.

The Nevsky Express high-speed train partially derailed on November 27, 2009, after a bomb equivalent to 7 kg (15 lbs) of TNT was detonated on the track. The attack took the lives of 28 people and left more than 90 injured.