A Taliban attack on a Shia mosque in the northwestern Pakistani city of Peshawar has killed at least 20 people and injured more than 65 others, government officials say.

At least four suicide bombers, wearing security forces uniforms, hit the Imamia mosque during Friday prayers, officials and witnesses said.

Three of the attackers succeeded in exploding their vests, while one was unable to do so, Shafqat Malik, the chief of Peshawar's police Bomb Disposal Unit (BDU), told Al Jazeera.

The unexploded vest was defused by the BDU, he added, while several grenades were also recovered from the site.

The Tehreek-e-Taliban Pakistan (TTP) claimed responsibility for the attack in a statement.

A spokesman for the Pakistani Taliban, Mohamad Khurasani, said the attack was in revenge for the execution of one of their members by the Pakistani government.

The group is also responsible for the December 16 attack on a school in Peshawar that left 150 people dead, mostly students.

Senior police official Rana Umer Hayat said several gunmen threw grenades into the mosque before storming the building. The attackers appeared to gain access to the mosque, which is under tight security, from an under-construction building next door.

"They cut razor wire on the boundary wall to gain access," provincial police chief Nasir Durrani said. Once inside, the attackers were engaged by the mosque's guards, as well as six policemen deputed to provided security to the building.

"The police and military forces that surrounded the area have now cleared the area, and are combing nearby areas for any attackers that may have escaped," Al Jazeera's Kamal Hyder reported from Islamabad, the Pakistani capital.

'A war state'

Helicopters were visible overhead, as security forces combed the streets around the blast site in a bid to find one of the attackers.

Asked whether there was a specific threat against this mosque, Durrani said: "The military operation Zarb-e-Azb [in North Waziristan] and in Khyber agency [against the Taliban] is ongoing. We are in a war state."

One of those killed on Friday, Naveed Abbas, worked for the UN Department of Safety and Security, according to a statement released by the UN.

Ban Ki-moon, UN secretary-general, condemned the attack and urged Pakistani authorities, "to redouble their efforts to protect religious and ethnic monitories and to combat terrorism in all its forms", the statement said.

This is the third attack against a Shia mosque since the attack on a Peshawar school killed more than 141 people on December 16.

The last attack took place two weeks ago, also during Friday prayers, at a Shia mosque in the town of Shikarpur, in Sindh province, killing at least 60 people.

Al Jazeera's Asad Hashim in Islamabad contributed to this report.

Source: Al Jazeera