Pakistan has told U.S. special envoy Marc Grossman that it is not possible at the moment for him to visit the country, a senior government official told Reuters on Wednesday, highlighting the increased tensions between the uneasy allies.

He did not elaborate on the reason for refusing Grossman's request to visit.

Relations between Islamabad and Washington plunged to the lowest point in years when a NATO cross-border air attack killed 24 Pakistani soldiers on November 26.

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.

Troubled U.S.-Pakistan ties threaten to set back peace efforts in neighbouring Afghanistan, where the United States is gradually withdrawing troops after a decade of war.

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

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.

(Writing by Serena Chaudhry; Editing by Chris Allbritton and Sanjeev Miglani)