By Phil Stewart and Missy Ryan
WASHINGTON (Reuters) - In what would be a major breakthrough, Pakistan and the United States appeared on Tuesday to be on the verge of clinching an agreement to reopen ground supply lines into Afghanistan, a U.S. official said, as Pakistan confirmed its president will attend a summit of NATO leaders this weekend in Chicago.
Ties between the United States and Pakistan have been severely strained over the past year, fuelling speculation Islamabad might be excluded from the high-level NATO talks on Afghanistan's future because of the failure to reach an agreement on the supply lines, which have been shut for months.
Pakistan closed down the supply lines for the Afghan war effort following the NATO air strike in November that killed 24 Pakistani soldiers. That strike fanned national anger over everything from covert CIA drone strikes to the U.S. incursion into Pakistani territory last year to kill al Qaeda leader Osama bin Laden.
Nadeem Hotiana, an embassy spokesman, confirmed that Pakistani President Asif Ali Zardari will attend the May 20-21 summit.
"This meeting will underline the strong commitment of the international community to the people of Afghanistan and to its future. Pakistan has an important role to play in that future," NATO spokeswoman Oana Lungescu said in a statement.
Afghan President Hamid Karzai is also expected to attend the meeting, where NATO nations will hone their plans to withdraw most of their troops by the end of 2014. As the Western presence ebbs, Pakistan, whose tribal areas are home to Taliban and other militants, will be key in shaping Afghanistan's future.
But the supply routes have been a major sticking point.
A U.S. official, speaking on condition of anonymity, said the deal now appeared almost certain, even while stressing that nothing had been finalized and surprises were always possible.
The official added that the U.S. military might see some higher costs to receive goods in Afghanistan than it did before Pakistan cut off the supply lines, but did not elaborate.
The United States and Pakistan, which now says it expects the routes to reopen, have long been expected to strike a deal that would include tariffs on NATO supplies going into Afghanistan.
After weeks of talks between U.S. and Pakistani officials in Islamabad, a Pentagon spokesman on Tuesday said he hoped that would occur in the "very near future."
In a statement, Prime Minister Yusuf Raza Gilani's office said ministers had backed a proposal to allow NATO to send only non-lethal equipment into Afghanistan on Pakistani roads.
"It was also decided that the military authorities should negotiate fresh border ground rules with NATO ... to ensure that (such border) incidents do not reoccur," the statement said.
(Additional reporting by Sebastian Moffett in Brussels and Sheree Sardar in Islamabad; Editing by Robin Pomeroy, Cynthia Osterman and Lisa Shumaker)