MAKHACHKALA, RUSSIA—Bombs hidden in two vehicles exploded outside a police station in the Russian republic of Dagestan, killing at least 13 people and wounding scores. Authorities said Friday it was probably a suicide attack by Islamic insurgents.

More than 130 people were hospitalized, 16 of them in critical condition, said Kazanfar Kurbanov, the Emergency Ministry chief physician in Dagestan.

Militants frequently attack police and civilians in Dagestan in Russia’s restive North Caucasus, and law enforcement is engaged in nearly daily clashes with local militants. But bombing attacks such as Thursday night’s in Makhachkala, the capital, are less common.

The first explosion occurred outside a traffic police station in Makhachkala when a car pulled up there for an ID check, officials said. Twenty-five minutes later, as the scene of the blast was filled with troops, investigators and firefighters, a second explosion occurred in a minivan parked nearby. Seven police officers and three firefighters were killed.

The two explosions were probably a co-ordinated attack aimed at police, said local Interior Minister spokesman Vyacheslav Gasanov. The first blast was equivalent to 30 kilograms of TNT, while the second one about 50 kilograms of explosives, he said.

Magomed Sultanov, a duty officer at the Interior Ministry in Makhachkala, told The Associated Press that investigators suspect the explosions were carried out by two suicide bombers. The remains of a man and a woman were found near the two vehicles.

Police reinforcements have been redeployed from neighbouring regions to Dagestan in recent weeks in an attempt to bolster security and make up for personnel shortages among local law enforcement forces.

Local media reported that as many as 20,000 troops would be dispatched to Dagestan. Those figures were dismissed by the president of the Dagestan province, Magomedsalam Magomedov.

He conceded, however, that his province was sorely lacking in security. Both Chechnya and Dagestan have 18,000 police officers, but Dagestan’s population is twice as large as Chechnya’s.

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