A team of Defense Department experts who arrived in Pakistan several weeks ago to negotiate the opening of key border crossings into Afghanistan are leaving this week after having made little-to-no progress.

Some of the negotiators left over the weekend, and the rest are expected to leave in the coming days, according to Pentagon spokesman George Little.  

Their departure follows Defense Secretary Leon Panetta's trip to the region, where he had harsh word for Pakistan. 

While in Kabul, Afghanistan, he told reporters the United States was "reaching the limits of our patience" with Pakistan.

Subsequently, Pakistani Army Chief Gen. Ashfaq Parvez Kayani refused to meet with one of Panetta's envoys -- Assistant Secretary of Defense Peter Lavoy. 

Though the negotiators are expected to return at some point in the future, it's unclear exactly when that might happen.

While the Pentagon is leading the effort, the State Department plays a supporting role in the negotiations to reopen the border crossings. 

The routes were closed last November by Pakistan in retaliation for a U.S. air assault that killed 24 Pakistani soldiers. The border crossings had been an important means of getting U.S. war materiel into Afghanistan. 

There have been a number of sticking points in the talks to reopen the border, including the Pakistani demand for an apology for the November deaths and the price that Islamabad will charge to allow convoys to pass through the country. Last week, NATO finalized agreements with other countries through which the U.S. has been sending supplies since Pakistan closed its routes.

The Associated Press contributed to this report.