At least three policemen were killed and six others injured when a police station came under attack by three militants in Pakistan's northwest city of Peshawar early on Friday morning, said a senior police officer in the city.

City Police Officer Imtiaz Altaf said that the three attackers were also killed by blowing themselves up in frustration after nearly two hours of exchange of fire with the police.

The attack came at about 7:00 a.m. local time when three militants armed with sophisticated weapons and wearing suicide jackets sneaked into a police station in the Lahore Gate area of the city from the rear gate of the building.

They first hid in the TV lounge of the police station and then opened fire and hurled handgrenades at the police inside the building, said Munawar Khan, a sub-inspector of the police station, who got injured in the attack.

Khan told local media at the hospital that there were an estimated 20 policemen inside the building when the police station was attacked. The injured sub-inspector later died at the local Lady Reading Hospital.

A large contingent of police and security forces were called in when the police station was attacked. They surrounded the police station occupied by the militants and exchanged fire with them for nearly two hours until the attackers blew themselves up inside the police station.

"They blew themselves up in frustration," said the City Police Officer Imtiaz Altaf. "We have found body parts of the three attackers," he said, adding that the three attackers are all aged below 20.

A bomb disposal squad have found seven handgrenades on the spot while clearing the site. They said that an estimated five to six kilograms of explosives were used by the suicide attackers.

No group has claimed responsibility for the attack yet.