WASHINGTON (AP) ― The U.S. team that tried for more than a month to negotiate a reopening of blocked Pakistani supply routes into Afghanistan is coming home without an agreement, U.S. officials said Monday.

Both sides indicated they remain open to making a deal, but the departure of the U.S. negotiators appeared to signal that the Americans see little prospect of a breakthrough any time soon. Adding to the appearance of an impasse was the Pakistanis’ refusal to grant a visiting Pentagon official a meeting with their top general.

White House press secretary Jay Carney put the onus on Pakistan to resume serious talks.

“We saw it as the right move to withdraw” the U.S. negotiating team, he said, adding that it had largely completed its work. “We are ready to send officials back to Islamabad when the Pakistani government is ready to conclude the agreement.”

Carney said several issues remain unresolved, but gave no details.

The disagreement over the supply routes is one of numerous tensions in the U.S.-Pakistani relationship. In recent days the Americans have emphasized their frustration at Pakistan’s refusal to do more to stop Pakistani-based insurgent groups from crossing into Afghanistan to fight U.S., Afghan and allied troops.

Officials in Washington and Islamabad would not detail what led to the break in the supply route talks, but two senior U.S. officials familiar with the negotiations said the Pakistanis were holding out for an apology for the deaths last November of 24 Pakistani soldiers in a U.S. airstrike, which was what triggered Pakistan to close the border. The officials spoke on condition of anonymity because of the sensitivity of the negotiations.

The U.S. has insisted it will do no more than express regret for the deaths and will not apologize.

One of the U.S. officials said the Pakistanis had put the apology demand “front and center” in the negotiations.

The border crossings had been an important means of getting U.S. war materiel into Afghanistan. Since then, NATO and the U.S. have been using a circuitous, more costly northern route to bring supplies to the war front.

Although the U.S. administration would like to have access to the Pakistani ground supply routes, officials have said they can continue the war and arrange the scheduled withdrawal of U.S. and allied troops and equipment without the routes.

Lt. Gen. Curtis Scaparrotti, the No. 2 U.S. commander in Afghanistan, on Monday acknowledged the troubled Washington-Islamabad relationship, but said the closed border has not affected his operations.

Scaparrotti told reporters at the Pentagon that commanders are “working very hard” to improve the military-to-military relationship between the two countries and get it back to where it was before the November killings.

“We’re a fair ways from that right now,” he said.

But Scaparrotti said the U.S. has built alternative supply routes and has been able to get what it needs for the war despite the border closing. NATO last week finalized agreements to move supplies through other countries.

“It’s not really affected us,” Scaparrotti said via a video teleconference from Kabul.

Pentagon press secretary George Little said the U.S. negotiators are prepared to return to Islamabad on short notice if circumstances change, and Pakistani officials said it would be wrong to conclude that talks have broken down.

“U.S. officials involved at the technical talks have completed their work,” Moazzam Ahmed Khan, the Pakistan Foreign Ministry spokesman, said. “And my understanding is that they have been recalled.”

State Department spokeswoman Victoria Nuland said it was unclear when the negotiators would go back to Islamabad.

“There’s no question that although we’ve made some progress, this remains difficult and we’re not finished,” Nuland said. Asked when talks might resume she said, “We need to hear from the Pakistanis when they think that’s a good idea.”

Little did not say who decided to pull out the U.S. team. “A decision was reached that it was time to bring the team home for a short period of time,” he said. “Again, we’re ready to send them back at any moment.”

Little also confirmed that a senior Defense Department official, Peter Lavoy, talked to senior Pakistani officials over the weekend but was not allowed to meet with Gen. Ashfaq Parvez Kayani, chief of the Pakistani army.

There have been a number of sticking points in the talks to reopen the border besides the issue of an apology for the 24 deaths. There have also been tough negotiations over the fee that Pakistan would charge for each truck to cross its territory. Before the November attack, the cost had been $250 per truck. As of late May, Pakistan was demanding $5,000 per truck and the U.S. had countered with $500. It’s unclear where that issue stood as of Monday.

Last week, Pakistani-U.S. relations seemed to hit a new low. Defense Secretary Leon Panetta visited India, Pakistan’s archrival, and Afghanistan and in each locale openly expressed frustration with the Pakistani’s government willingness to help the U.S. in the war on terror, and acknowledged aloud that “the whole idea” was to leave Pakistan in the dark about the secret raid that killed Osama bin Laden in an Army garrison town in Pakistan last year.