Explosions have targeted two police stations in the northwest Nigerian town of Sokoto, witnesses and police say.

A senior police officer, speaking on condition of anonymity, told the AFP news agency that a suicide bomber had approached the Yan Marina police station in the centre of the city on Monday.

After he was denied entry at the security gate, the vehicle blew up, the officer said.

Usman Bube, a resident of the area, said the area was "engulfed in flames".

"There was a huge explosion about an hour ago at the police station that was followed by thick smoke that enveloped the area," said Bube.

Lawai Danfili, another resident, reported the second blast near the Unguwar Rogo station, while other residents of the town reported seeing black smoke rising in the air from the direction of the two explosions.

"We had a loud explosion around the police station. The impact of the explosion shook houses in the area. We all moved indoors," he said.

Police and security agents have blocked off roads to the sites of the blasts.

The explosions come as Nigeria faces an increasing threat from Boko Haram, an armed group that has frequently targeted government and police installations.