The explosion occurred after a big truck loaded with explosives entered the FSB - formerly the KGB - premises in the newly established regional capital of Magas, an Interior Ministry spokesman said in Moscow.

At least one person died and 30 were injured, according to the ITAR-TASS news agency, but Interfax said three people were killed and 24 were hurt.

Among the dead was Mikhail Safonov, a deputy director of the FSB branch in Chechnya, Interfax reported.

"There are reports of numerous casualties," Interfax quoted local Interior Ministry spokesman Yakhya Khadziyev as saying.

"There is no exact number of killed and wounded, but there are many of them," another government official told Interfax news agency following the blast in the Ingushetia region.

Truck bomb

The attack occurred shortly before noon on Monday when an explosives-laden truck pulled up next to the building, inside which were at least 100 people, news agencies reported.

Another report said that a bomb had been placed on the roof of the four-storey building in the town of Magas.

Tens of thousands of Chechen refugees have been living in Ingushetia after fleeing fighting in their republic.

Ingushetia borders Chechnya, where federal troops have been fighting separatists for nearly four years, the second such war in a decade.

Increase in attacks

Tens of thousands of Chechen refugees have been living in Ingushetia after fleeing fighting in their republic.

Chechen separatists had vowed to increase their attacks on Russian targets ahead of Kremlin-organised presidential elections in the republic on 5 October.

The Magas blast comes in the wake of a similar attack on 1 August, in which a truck bomb killed over 50 people and injured more than 80 others at a military hospital in the city of Mozdok in the Russian republic of North Ossetia.