A truck packed with explosives rammed through the gates of a police compound in southern Russia on Monday and exploded in a suicide attack that killed at least 20 people and wounded around 130, officials said.

The attack in the city of Nazran, the main city in Ingushetia, occurred as police officers lined up for roll call at the start of their morning shift. It killed and wounded officers in the compound and local residents in homes nearby, officials said.

A total of 138 people sustained injuries in the blast, including 10 children between five and 12 years old, said a regional spokesman for the emergency situations ministry in Rostov-on-Don, Oleg Grekov.

In a move underscoring the seriousness of the situation, President Dmitry Medvedev announced hours after the attack that he had sacked the region's top policeman and issued a stern command to his interior minister to bring about order in law enforcement in the region.

"This terrorist act could have been avoided," a stern-faced Medvedev said on state television.

He told federal Interior Minister Rashid Nurgaliyev to submit to the Kremlin "concrete proposals on how to bring about order and strengthen cadres within Ingushetia's interior ministry".

Medvedev's press office said he offered his condolences to the families of the victims.

"This crime is further proof of the lack of humanity of these gangsters who are pursuing their attempts to destabilise Ingushetia ... We will do everything to find and punish the perpetrators of this cruel crime."

Late Monday, rescue teams were still searching for more people that might be buried under the debris, Grekov told AFP. He refused to speculate how many people could be found.

Twenty people have died and 74 were hospitalized, said a spokeswoman for local police, Madina Khadziyeva.

Many of those injured were from residential buildings adjacent to the police station, officials said.

The blast site was a scene of devastation with overturned cars, twisted rubble and burned trees littering the ground under a thin pall of grey smoke and the shattered windows of the nearby apartment buildings.

The attack, four days after the Kremlin said Ingushetia's President Yunus-Bek Yevkurov was returning to work despite still recovering from injuries sustained in a bomb attack in June, underscored growing fears over stability in the region.

Local officials declared three days of mourning, beginning Monday.

Investigators said a Russian-made Gazelle light truck had crashed through the security gate at the Nazran police headquarters and sped into the building's interior courtyard.

Local officials said the truck blew up near the police armament depot, setting off explosions of ammunition that shook the area well after the initial blast and that hampered search and rescue operations.

The emergency situations ministry dispatched a large plane to the airport at Beslan, nearby site of a 2004 school hostage massacre, and officials said those most seriously hurt in Monday's attack would be evacuated for treatment at hospitals in Moscow.

Violent attacks by militants on Russian law enforcement personnel have become an almost daily occurrence in Ingushetia, but Yevkurov's spokesman, Kaloi Akhilgov, said Monday's blast was extraordinary.

"We have not had such an attack for a long time," he said, adding: "There is not enough blood" to treat the injured.

Yevkurov, who was nearly killed in an attempt on his life on June 22, blamed Islamist militants for the attack.

"The militants have carried out this terrorist act with the aim of destabilizing the situation in the republic," he said, in comments relayed by his spokesman.

US President Barack Obama said he was "deeply troubled" by the attack.

"There can be no justification for such an act of terrorism. This latest attack highlights the concerning increase in violence in the region affecting officials and civilians alike. Our condolences go out to the government of Russia and the families of victims," he said.

The European Union called the attack "a brutal act" of terrorism.

Ingushetia is one of seven administrative territories known as "republics" that constitute the North Caucasus region in southern Russia, long the most unstable part of the country.

Last Wednesday, the republic's construction minister, Ruslan Amerkhanov, was shot dead in a brazen attack in his office.

The spectacular attacks have fuelled fears that the situation in Ingushetia, which neighbours Chechnya, was quickly getting beyond the control of federal authorities.

Moscow has struggled to impose the Kremlin's authority in the volatile North Caucasus region, which has been the site of two full-fledged wars in Chechnya and countless attacks since the 1991 collapse of the Soviet Union.