Pakistan Severing Ties with United States?

Pakistan has rejected U.S. special envoy Marc Grossman's request to visit the country, a senior official said on Wednesday, highlighting the increased tensions between the uneasy allies.

He did not elaborate on the reasons.

Pakistan's Foreign Minister Hina Rabbani Khar (L) gestures to U.S. special representative Marc Grossman after a photo opportunity at the foreign ministry in Islamabad October 13, 2011.

Ambassador Grossman asked to visit Pakistan but we conveyed to him that it was not possible at the moment, a senior government official, who declined to be named, told Reuters.

Relations between Islamabad and Washington are at the lowest point in years, dragged down by a NATO cross-border air attack which killed 24 Pakistani soldiers on November 26.

The growing tension threatens to set back peace efforts in neighbouring Afghanistan, where the United States is gradually withdrawing troops after a decade of war.

Pakistan's cooperation is regarded as crucial, because of its long history of association with militant groups, to efforts to persuade the Taliban to join negotiations.

Grossman, U.S. Special Representative to Afghanistan and Pakistan, is due to visit Afghanistan and Qatar this week, U.S. Secretary of State Hillary Clinton said last Wednesday.

Pakistan said in early December it had decided to review cooperation with the United States and NATO. The review is currently before parliament with no firm timeline on when recommendations will be presented to the government.

U.S. State Department spokesman Mark Toner said on Tuesday Pakistan had decided the review should be completed before Grossman's next visit.

Ties between Washington and Islamabad were severely hurt in January 2011 by the killing of two Pakistanis by a CIA contractor.

The United States further infuriated and embarrassed Pakistan's powerful military in May with a unilateral special forces raid that killed al Qaeda leader Osama bin Laden.

Pakistan said the raid, of which it wasn't informed, was a violation of its sovereignty.

Relations between Pakistan's civilian leadership and military are also at their worst since a 1999 coup following reports of a disputed memo allegedly from President Asif Ali Zardari's government seeking U.S. help in reining in Pakistan's powerful generals.