U.S.-Pakistan relations plunged into crisis on Saturday when NATO helicopters and fighter jets killed as many as 28 Pakistani troops in attacks on two military outposts in northwest Pakistan.

Pakistan retaliated by shutting NATO supply routes into Afghanistan that are vital to the 10-year war against the Taliban and asked the United States to vacate within 15 days an air base used by U.S. forces for drone strikes, which have been at the centre of a dispute between Islamabad and Washington.

Following are the main events this year that have strained ties between Washington and Islamabad:

BIN LADEN

Tensions between the United States and Pakistan were exacerbated this year by a unilateral raid by U.S. special forces that killed al Qaeda leader Osama bin Laden in Pakistan in May, humiliating Pakistan's military.

Pakistan, which uneasily allied itself with Washington after the September 11, 2001 attacks on the United States, called the May 2 raid a flagrant violation of its sovereignty.

The raid further deepened suspicions in Washington's prickly relations with a strategic ally in the U.S.-led war on terrorism. Pakistan's army warned the United States that it would put cooperation at risk if it conducted another assault.

Even before the raid, Pakistan regularly complained about U.S. drone strikes that have killed hundreds in Pakistan's tribal regions. Pakistan says the strikes undermine efforts to deal with militancy because civilian casualties inflame public anger and bolster support for the fighters.

The fact that bin Laden was found in Pakistan angered many U.S. lawmakers who questioned why the United States gives so much aid to Pakistan when the leader of al Qaeda, the group blamed for the 2001 attacks on New York and Washington, was able to live with impunity in the country.

No warning of the bin Laden raid was given to Pakistan, illustrating the depth of U.S. officials' mistrust of their Pakistani counterparts. The operation was seen as humiliating to the Pakistani military because U.S. helicopters were able to slip in and out of their territory from Afghanistan apparently without the knowledge of Pakistani forces.

MULLEN SLAMS ISI

On September 22, U.S. Joint Chiefs of Staff chairman Admiral Mike Mullen angered Pakistan by accusing its ISI military intelligence agency of sponsoring the Haqqani network, one of the most lethal Taliban-allied groups fighting U.S.-led forces in Afghanistan.

The outgoing U.S. military chief said the the militant group was a veritable arm of the ISI. The fact remains that the (Taliban) and the Haqqani Network operate from Pakistan with impunity, he told the Senate Armed Forces Committee.

Pakistan's government issued vehement denials, but Mullen's comments strained relations.

The White House urged Pakistan to break any links it has with the Haqqani network and take action to shut down their safe havens along the Afghanistan-Pakistan frontier.

CIA CONTRACTOR

Relations between the intelligence establishments of the two countries have been on a downward spiral since January after a CIA contractor killed two Pakistani men he said were trying to rob him in Lahore on January 27.

Joint operations against militants were suspended soon afterward.

Pakistan had railed against the presence of U.S. special forces and CIA operatives in the country and relations reached a new low with the jailing of the CIA contractor, Raymond Davis, 36, a former U.S. special forces member.

Davis' arrest sparked a tense diplomatic stand-off that ended only after the families of those killed were paid blood money compensation. The issue severely impacted cooperation between the CIA and Pakistan's ISI spy agency.

KHYBER PASS CLOSURE

Pakistan has closed NATO's vital supply route through its territory before, for 10 days, following a similar incident on Sept 30, 2010, in which NATO forces killed two Pakistani service personnel.

Trucks and fuel tankers for foreign forces in Afghanistan were stopped at the Torkham border post in the Khyber tribal region near the city of Peshawar, hours after the raid.

NATO apologized for the incident, which it said happened when its gunships mistook warning shots by Pakistani forces for a militant attack.

(Compiled by Anthony Boadle)