Grenade-toting Taliban militants stormed a Shiite mosque in northwest Pakistan on Friday, police said, in an attack that left at least 18 people dead.

The incident comes two weeks after a suicide bombing at a Shiite mosque in southern Pakistan killed 61 people, the deadliest sectarian incident to hit the country in nearly two years.

Police said several gunmen threw grenades before storming the Imamia mosque in Peshawar, the main city in Pakistan's restive northwest, around the time of the main Friday prayers.

An AFP reporter saw 18 dead bodies in the Hayatabad Medical Complex, while the hospital's public relations officer Toheed Zulfiqar put the toll at 19, saying one of the dead may have been an attacker.

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

Senior police official Mian Saeed said the assault, which left more than 60 wounded, began when the militants entered the mosque from a nearby building site.

"They were wearing suicide vests and carrying grenades and Kalashnikovs," Saeed told AFP.

"One blew himself up while another was shot by police and was later killed. The operation inside the mosque is over but we are conducting a search operation in the surrounding buildings," he said.

TV footage showed people running away from the scene, some carrying injured on their shoulders, others limping, as police fired shots and checked people at a barrier.

Witness Muhammad Raza told AFP: "There was a huge explosion, I can see many injured lying in front of me."

An AFP reporter at the scene saw soldiers and police commandos arriving.

The mosque is close to several government buildings including the offices of the Federal Investigation Agency and passport agency.

Mushtaq Ghani, information minister for Khyber Pakhtunkhwa province, of which Peshawar is the capital, told Geo TV the attack was a response to the army's ongoing offensive against militants in the tribal areas along the Afghan border.

Since June last year the army has been waging a major campaign against strongholds of Taliban and other militants in the North Waziristan tribal area, which lies close to Peshawar.

The military has heralded the success of the operation, which it says has killed more than 2,000 militants, though the precise number and identity of those killed cannot be verified independently.

Pakistan has suffered a rising tide of sectarian violence in recent years, most of it perpetrated by hardline Sunni Muslim groups against minority Shiite Muslims, who make up around a fifth of the population.

The suicide bombing at a mosque in southern Sindh province on January 30 was the deadliest sectarian attack in Pakistan since February 2013, when 89 were killed in a market bombing in the southwestern city of Quetta.

Anti-Shiite attacks have been increasing in recent years in Karachi, Quetta, the northwestern area of Parachinar and the far-northeastern town of Gilgit.

The country has stepped up its fight against militants since a Taliban school massacre in Peshawar in December.

Heavily armed gunmen went from room to room at the army-run school gunning down more than 150 people, most of them children, in an attack that horrified the world.

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