A team of U.S. negotiators is returning home from Pakistan after several weeks of talks aimed at re-opening vital supply routes to NATO-led forces in Afghanistan ended with no progress, officials in both countries said on Monday.

The United States is trying to convince the Pakistani government to reopen its key ground routes into neighboring Afghanistan which were closed in November 2011 after a cross-border NATO air raid accidentally struck military posts and killed 24 Pakistani soldiers, bringing U.S.-Pakistani relations to a new low.

Pentagon spokesman George Little said a part of the U.S. team already left Islamabad over the weekend, and the rest will return shortly. "We've not reached a resolution yet with the Pakistanis on reopening the ground supply routes," he said. "We hope to resolve the issue soon."

But officials in both Islamabad and Washington insisted the talks have not failed. "We will continue to have dialogue, so while the issue is not resolved, the talking has not stalled," Little said, adding that the United States will continue to work "very hard" on the issue with their Pakistani counterparts.

Several issues have prevented both sides from reaching an agreement, including Pakistan's continuing demands for an official apology for the deaths of the Pakistani soldiers. Washington has acknowledged the deaths but stopped short of an official apology, angering officials in Islamabad.

The multinational alliance is currently forced to use alternative supply routes to Afghanistan through Central Asia, the Caucasus and Russia. NATO signed deals last week with Kazakhstan, Uzbekistan and Kyrgyzstan to use their territory to evacuate vehicles and military equipment from Afghanistan.