A suicide bomber rammed the gates of the local police headquarters in the city of Nazran in Russia's North Caucasus on Monday. (Associated Press)

A suicide bomber exploded a truck at a police station in Russia's restive North Caucasus Monday, killing at least 20 people and wounding about 100 others, officials said.

The bombing was the deadliest for months in Russia's south and dented Kremlin claims to be stabilizing the North Caucasus region, where 15 years of separatist fighting in Chechnya has increasingly spilled into surrounding provinces.

The attacker rammed the gates of the local police headquarters in the city of Nazran in Ingushetia and detonated his explosives as police officers lined up for a morning check, said Kaloi Akhilgov, a spokesman for the regional president.

At least 20 people were killed, said Svetlana Gorbakova of the regional branch of the investigative committee of the Russian Prosecutor General's office.

Victims transported to Moscow

Figures on the number of people injured ranged from 92 to 118. There was no immediate explanation for the discrepancy in numbers.

Officials said about 100 people had to receive medical treatment and at least 10 people were in critical condition. Some of the wounded were being transported to Moscow for treatment, officials said.

The attacker and the truck, which carried at least 20 kilograms of explosives, were pulverized by the blast, Gorbakova said.

A nearby apartment building and several office buildings were also damaged, and burned-out cars littered the street.

The blast left a huge crater and triggered a raging fire that destroyed a weapons room where ammunition detonated.

It took rescue teams several hours to search for victims in the rubble.

Government officials alleged Monday's suicide attack had been organized by militants trying to avenge recent security sweeps in the forests along the mountainous border between Chechnya and Ingushetia.

While large-scale fighting from the two wars that ravaged Chechnya since 1994 has ended, Islamic militants continue to mount regular hit-and-run attacks and skirmishes. Bloodshed has surged in recent months and increasingly spilled into Chechnya's neighbours.

Ingushetia and other provinces in the region have been destabilized by shootings, bombings and other attacks by militants targeting police and government officials.