The United States has withdrawn negotiators from Pakistan after talks failed to reopen vital NATO supply routes into Afghanistan, officials said Monday.

The move signaled further strain in troubled Pakistani-US relations and followed harsh criticism last week from US Defense Secretary Leon Panetta that saw Pakistan's army chief refuse to meet a senior Pentagon official.

The negotiators had been in Pakistan for about six weeks, as US officials believed they were close to a deal with Islamabad to lift the blockade.

Pakistan shut its border to NATO supply convoys on November 26 after a botched US air strike killed 24 Pakistani soldiers.

But no breakthrough was imminent and there was no scheduled date for a resumption of the talks, Pentagon spokesman George Little told reporters.

"The decision was reached to bring the team home for a short period of time," Little said.

But Washington has not given up on discussions with Islamabad, he said.

"That's not to be taken as a sign of our unwillingness to continue the dialogue with Pakistanis on this issue," he said, adding the negotiators are "prepared to return at any moment."

Members of the negotiating team, which included officials and legal advisers from the State and Defense departments, started to leave over the weekend and the remainder would soon return to the United States, Little said.

Pakistan's army chief, General Ashfaq Kayani, refused last week to meet senior Pentagon official Peter Lavoy, who traveled to Pakistan to try to resolve the dispute, officials said.

Lavoy, principal deputy assistant secretary of defense for Asian and Pacific security affairs, "was hoping to meet with General Kayani to work through this issue," Little said.

The roads through Pakistan are a crucial logistical link for NATO as it plans a withdrawal of most of the remaining 130,000-strong combat force in Afghanistan, along with vehicles and equipment, by the end of 2014.

But Washington has refused to apologize for the November air raid, infuriating Pakistan, and US officials have refused to pay several thousand dollars for each truck crossing the border, as reportedly demanded by Pakistan.

The White House said an agreement would be reached when Pakistan is ready.

"Most of the technical agreements have been worked out but there are still several issues outstanding. We believe that all can be resolved and we remain ready to conclude this agreement as soon as Pakistan is ready," spokesman Jay Carney said.

Pakistan sought to downplay the departure of the US team of "technocrats".

"There is no deadlock or a stalemate," foreign ministry spokesman Moazzam Ahmed Khan told AFP.

"They were involved in technical discussions and gave their input... We have negotiations with the US at different levels, including the political level, and those negotiations are continuing," he added.

Pakistan's ambassador to the United States, Sherry Rehman, said the border crossing was not closed "in a fit of pique or on impulse" and that 24 Pakistani soldiers had been killed in the US air strike, "absent an expression of remorse."

With the Pakistani roads shut, the US-led NATO force has relied on cargo flights and a network of northern road and rail routes -- negotiated with Russia and governments in Central Asia and the Caucasus.

But the northern routes are much longer and more expensive than the roads through Pakistan.

"As a technical matter, we could in theory do our work without the ground supply routes. It would certainly be better to have them open and less costly," Little said.

Relations have got steadily worse between Pakistan and the United States, supposed allies in the war in Afghanistan and against Al-Qaeda.

Panetta warned Pakistan on Thursday that the United States was running out of patience over its refusal to eliminate sanctuaries for insurgents, who attack US troops in Afghanistan.

Relations plunged to an all-time low in May 2011 when US Navy commandos killed Al-Qaeda leader Osama bin Laden in his compound in a Pakistani garrison town.

The Pakistanis were incensed not to have been briefed on the raid before it happened.