A suicide bomber has attacked police headquarters in the northeast Nigerian city of Maiduguri, killing six people, a hospital official said Friday amid warnings that the situation in the city is worsening.

A hospital official said the bodies were brought Friday to the hospital from outside the police headquarters in Maiduguri. Sixteen people were hospitalized with injuries from the blast, the official on condition of anonymity because he is not allowed to speak to press.

The attack occurred Friday around noon at the gate of the headquarters, said Borno State police chief Bala Hassan. The bomber had tried to enter the compound but was stopped by officers, Hassan said. Hassan said that only one policeman and two civilians were killed and six civilians were injured. However, officials have downplayed casualty figures in the past.

Hours earlier, a suspected bomber died when an explosive went off prematurely in another part of the city, said military spokesman Col. Victor Ebhaleme.

No group immediately claimed responsibility for Friday's attacks, although a radical Islamist sect known as Boko Haram has carried out similar attacks.

The U.S. Embassy issued a warning late Thursday to its citizens not to travel to Maiduguri as diplomats believe "the situation will continue to deteriorate."