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 coordinated attack aimed at police, said local Interior Ministry spokesman Vyacheslav Gasanov. The first blast was equivalent to 66 pounds of TNT, while the second one about contained 110 pounds 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.

Russian Interior Minister Rashid Nurgaliyev said in remarks carried by the NTV television station that the insurgents could have targeted the police station and its personnel -- or might have been driving the explosives to downtown Makhachkala in order to stage a bombing there during the May 9 Victory Day parade.

Police reinforcements have been redeployed from neighboring 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 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.

In Brussels, EU foreign policy chief Catherine Ashton condemned "the heinous terrorist attacks," offered her condolences to the victims, and said "there can be no justification for such criminal acts."