By Phil Stewart
WASHINGTON, June 11 (Reuters) - The United States said on Monday it was withdrawing its team of negotiators from Pakistan without securing a long-sought deal on supply routes for the war in Afghanistan, publicly exposing a diplomatic stalemate and deeply strained relations that appear at risk of deteriorating further.
Pakistan banned trucks from carrying supplies to the war effort in neighboring Afghanistan last year to protest a cross-border NATO air attack that killed 24 Pakistani soldiers, a measure U.S. officials initially hoped would be short term.
That strike fanned national anger over everything from covert CIA drone strikes to the U.S. incursion into Pakistan last year to kill al Qaeda leader Osama bin Laden, and the supply routes evolved into a lightning-rod issue between the two countries.
After six weeks of negotiations that at least once appeared close to a deal, the Pentagon acknowledged that the team had failed to clinch an accord and was coming home.
"I believe that some of the team left over the weekend and the remainder of the team will leave shortly," Pentagon spokesman George Little told reporters. They could return to Pakistan at any time, if warranted, he added.
With the Pakistan routes unavailable, NATO has turned to countries to the north of Afghanistan for more expensive, longer land routes. Resupplying troops in Afghanistan through the northern route is about 2-1/2 times more expensive than shipping items through Pakistan, a U.S. defense official told Reuters, speaking on condition of anonymity.
The announcement about the negotiators came just days after Defense Secretary Leon Panetta said the United States was reaching the limits of its patience because of safe havens Pakistan offered to Islamist insurgents, who are attacking U.S. forces across the border in Afghanistan.
Pakistan's envoy to the United States had warned that Panetta's comments last Thursday in Kabul were unhelpful to efforts to narrow the differences between the two countries and came at a critical moment in negotiations.
With U.S. negotiators returning home, White House spokesman Jay Carney suggested it was now up to Pakistan to break the deadlock.
"We are ready to send officials back to Islamabad when the Pakistani government is ready to conclude the agreement," Carney told reporters. "And it certainly remains our goal to complete an agreement as soon as possible."
State Department spokeswoman Victoria Nuland echoed those remarks and said "we've had some agreement in some areas."
"I think both sides are going to take some counsel and then we'll see when we can get back to it," she said.
Pakistan's ambassador to the United States, Sherry Rehman, said she did not view the decision to withdraw the negotiators as an "institutional pullout" by the United States.
The United States has rebuffed Pakistan's demands for an apology over the NATO air strike and both sides failed to agree on tariffs for supplies passing through Pakistan.
The Pentagon acknowledged on Monday that Pakistan's powerful army chief, General Ashfaq Kayani, declined a meeting last week with a top Pentagon official, Peter Lavoy. "He (Lavoy) was hoping to be able to meet with General Kayani to work through this issue," Little said.
Lieutenant General Curtis Scaparrotti, the No. 2 U.S. commander in Afghanistan, predicted the United States could carry out its planned withdrawal of most of its troops by the end of 2014, even without a deal with Pakistan on ground supply routes.
"It's not really affected us, and I don't expect it to be a problem here in the future," Scaparrotti, in Afghanistan, told Pentagon reporters in a video briefing.
But beyond the cost, the split with Pakistan is a worrisome sign that even seemingly straight-forward commercial agreements between the two countries are elusive. That bodes ill for agreement on other efforts, like tackling militant safe havens, that U.S. officials feel are fundamental for Afghanistan's long-term stability.
Panetta last week urged Pakistan to go after the Haqqani militant network, one of the United States' most feared enemies in Afghanistan, and said Washington would exert diplomatic pressure and take any other steps needed to protect its forces - remarks that sounded alarms in Islamabad.
The United States blames the group for a June attack on a U.S. base in the east in which several insurgents, including some wearing suicide vests, used rocket-propelled grenades.
The attack was foiled, but it underlined the challenge facing Western and Afghan forces in the east where insurgents take advantage of the steep, forested terrain and the Pakistani border to launch attacks and then slip back, commanders say.
Scaparrotti said the United States could still reach its objective of handing over security responsibility to Afghan forces even if Pakistan fails to go after Haqqani safe havens.
"I think we can still attain our withdrawal goals. And I also believe, while very difficult, we can attain our objectives of (an Afghanistan) secured by Afghans in 2014," he said. (Additional reporting by Laura MacInnis and Andrew Quinn; Editing by Will Dunham)