A suicide bombing at a mosque during Friday prayers in the northwestern Pakistani city of Peshawar has killed at least 30 people, police and hospital officials said.

The death toll is expected to rise substantially as many of the injured are in critical condition, police and hospital officials said. The authorities have not said who may be behind the attack.

"We are in a state of emergency and the injured are being shifted to the hospital," police officer Mohammad Sajjad Khan told Reuters.

Mohammad Aasim, spokesman of the Lady Reading Hospital, where victims have been brought, told Reuters they had received more than 30 bodies.

Senior police official Ijaz Khan confirmed that at least 30 had been killed and that it was a suicide bombing.

He told Reuters that two armed men arrived near the mosque on a motorcycle and were stopped for a search by police on duty outside.

"They opened fire on the police...and entered the mosque," he said.

Police are still determining if both had carried out suicide attacks inside the mosque.