ISLAMABAD, Pakistan — Pakistani Islamist militants on Sunday pledged to cease their four-year insurgency against Pakistani security forces, and join the Taliban's war against NATO troops in Afghanistan.

The agreement reunited four major Pakistan-based militant factions under the flag of Mullah Mohammed Omar, the Taliban chief, an announcement by the militants said.

Security experts in Islamabad said the agreement to end the insurgency with Pakistan was a dual-purpose tactical move by the Taliban.

It has lost hundreds of fighters during a two-year surge of U.S. forces in its southern Afghanistan strongholds.

The Pakistani militants, too, have been pummelled by security forces since 2009, and by late 2011 had splintered into dozens of factions without a unified command. The agreement coincided with discrete negotiations between the Pakistani militants and the government in Islamabad, held since October.

The pact would enable Mullah Omar to reinforce the Taliban ranks, while the pledged cessation of attacks against the Pakistani security forces would allow the militants greater freedom to launch cross-border attacks into Afghanistan.

"It will take a lot of pressure off the militants, and deepen the tensions between the U.S. and Pakistan," said Mansur Mahsud, director of research at the Fata Research Center, an independent think tank. "There will be angry complaints by the Americans, and counter-accusations by Pakistan that NATO isn't stopping raids by Pakistani insurgents from Afghan territory."

Taliban sources said three heavyweight militants mediated the intra-militant pact, reached after a month-and-a-half of reportedly tense negotiations: Abu Yahya al Libbi of al Qaida, and Maulana Mansoor and Siraj-ud-Din Haqqani of the Taliban.

The agreement bound together the factions, which previously had occasionally fought each other over territory, into a consultative council based in the twin Pakistani tribal regions of North and South Waziristan.

The regions, notorious as Taliban safe havens, are under constant surveillance by U.S. intelligence and, since 2004, have been the focal point of CIA drone-launched attacks.

The drone warfare has increased tensions, largely over contentions that innocent civilians have died on those attacks.

Meanwhile, relations between Pakistan and the U.S. hit rock bottom following the killings of 25 Pakistani troops by American forces in November in a friendly fire incident on the border with Afghanistan.

Pakistan was infuriated further when a Pentagon investigation, which it had declined to join, found that Pakistani troops had fired first in the incident, which was blamed on poor coordination.

For some time now, Pakistan has resisted American pressure to launch military operations against Afghan militants and their allies in North Waziristan, saying the 147,000 troops it has deployed to the tribal areas are overstretched.

However, U.S. officials have repeatedly asserted the reluctance reflects a covert alliance between Pakistan's security forces and the Taliban — in particular, the Haqqani Network, which draws fighters from the Waziristans.

The network brokered a peace agreement between its allies and the Pakistani security forces in 2006, ending two years of fighting.

The Waziristan council's first order of business was to reassert the Taliban's writ over Pakistani splinter groups, according to a pamphlet distributed in North Waziristan over the weekend.

Militants were warned to stop kidnapping Pakistanis for ransom, and to cease summary executions of tribesmen suspected of collaborating with the security forces.

"If any holy warrior is found involved in an unjustified murder or crime, he will be answerable to the council and could face Islamic punishment," the pamphlet declared.

This comes amid deliberations between American and Pakistani officials on the proposed opening of a Taliban office in Qatar, to facilitate peace talks.

Ahmed Pasha, chief of the Pakistani military's premier spy agency, the Inter Services Intelligence directorate, visited Doha last week for talks about the provision of an "address" to the Taliban.

Yousaf Gilani, the Pakistani prime minister, is to follow him on Wednesday.

(Hussain is a McClatchy special correspondent)

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