The United States has withdrawn negotiators from Pakistan after talks failed to produce a deal on reopening vital NATO supply routes into Afghanistan, officials said Monday.
NATO oil tankers stand parked near oil terminals in Pakistan's port city of Karachi.
The stalled negotiations signaled further strain in Washington's troubled relations with Pakistan and followed harsh criticism of Islamabad last week from US Defense Secretary Leon Panetta.
In another indicator that the talks on the supply lines had collapsed, Pakistan's army chief rebuffed a request for a meeting from a senior Pentagon official, officials said.
The negotiators had been in Pakistan for about six weeks, as US officials had believed they were close to a deal with Islamabad to lift the blockade.
Pakistan shut its border to NATO supply convoys in November 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.
The United States, however, would continue to maintain a "dialogue" with Pakistan and the departure of the expert negotiating team did not mean Washington had 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.
The departure of the negotiators came after 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 US officials have so far rejected Pakistani proposals to charge steep fees of several thousand dollars for each alliance truck crossing the border.
Washington has also refused to issue an explicit apology for the November lethal air raid.
The White House declined to be drawn into a discussion as to why Pakistan had yet to agree to reopen the supply route.
"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's ambassador to the United States, Sherry Rehman, said "we are still optimistic that we can reach a mutually satisfactory resolution to these issues."
She 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.
The deadlock over the supply lines is just one of several disagreements that have fueled tensions between the two countries, supposed allies in the US battle against Islamic extremists.
Panetta warned Pakistan on Thursday that the United States was running out of patience over Islamabad's refusal to do more to eliminate sanctuaries for insurgents, who attack US troops in neighboring 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 that they learned of the raid only after it had been carried out.