John Kerry's "engagement will be very critical to advancing our shared fight against militant extremism," a State Department official said.

AFP/Islamabad

US Secretary of State John Kerry arrived on Monday on an unannounced visit to Pakistan for key talks, only weeks after the country's worst ever militant attack left 150 dead in a school.

"The secretary's engagement will be very critical to advancing our shared fight against militant extremism," a senior State Department official said, adding Kerry would chair annual strategic talks between the two countries.

Last week's Islamist attacks in Paris have sharpened the global focus on militant extremism, although governments have been on high alert since a series of brutal attacks around the world last year.

Pakistan has strengthened its anti-terror strategy since the December 16 Taliban attack on an army-run school in Peshawar which killed 134 children and 16 adults and sparked global outrage.

Schools across Pakistan including the one in Peshawar reopened on Monday after an extended break following the assault.

Restive northwest

There have been a series of Pakistani airstrikes in the restive northwest, a hiding place for different regional militant groups, in the wake of the attack which Islamabad has described as its equivalent of the September 11, 2001 assault.

Kerry at the time along with President Barack Obama led global condemnation, calling the attack "gut-wrenching".

The State Department official, who asked to remain anonymous, said there was an "intensifying conversation on Pakistani counter-terrorism operations in North Waziristan and elsewhere" following the attack.

While there has been good cooperation in the fight against Al-Qaeda militants, the US wanted to "ensure that actions are met with a real and sustained effort to constrain the ability" of other groups such as the Haqqani Network, as well as the Pakistani Taliban, or Tehreek-e-Taliban Pakistan (TTP) which claimed the school attack, and the Afghan Taliban.

Kerry "will meet with Prime Minister Nawaz Sharif and co-chair the US-Pakistan Strategic Dialogue Ministerial with Pakistan's Adviser to the Prime Minister on National Security and Foreign Affairs Sartaj Aziz", a short statement by the State Department said.

A weekend online video purportedly showed dozens of former militants of the Pakistani Taliban pledging allegiance to the Islamic State group, which has seized a swathe of territory in Iraq and Syria.

The video raised the spectre of the IS group tapping support within Pakistan and Afghanistan, two nations with dozens of militant groups.

Washington has pressed Islamabad for years to wipe out the militant sanctuaries in lawless tribal areas such as North Waziristan, which have been used to launch attacks on Nato forces in neighbouring Afghanistan.

Many believe Pakistan's security services see the Haqqanis as an "asset" and maintain close links with them. One senior US official once described them as a "veritable arm" of the country's ISI spy agency.

'Credit due'

"We'll be talking about the elimination and not distinguishing between any terrorist groups," the official said.

"It's obviously no secret that the US has pushed Pakistan to do far more on counter-terrorism."

"But I also think that the government of Pakistan deserves credit for moving pretty decisively both after Peshawar and in the actions it has undertaken in North Waziristan," the official said.

Kerry also wanted to discuss ways to improve Afghan-Pakistani ties as well as reconciliation efforts with Taliban militants in each country.

Recent violence along the border in Kashmir between India and Pakistan would also be high on the agenda.

Kerry arrived after a short visit to India, where he attended an investment conference in the western state of Gujarat alongside new Indian Prime Minister Narendra Modi.