KABUL — Afghanistan and the United States are due to begin talks on Thursday on an agreement that will define how many American troops stay in the country after most NATO combat forces leave at the end of 2014, and the scope of their mission.

The talks on the bilateral security agreement could take months, and are expected to be difficult. The thorniest issue is whether U.S. soldiers in Afghanistan are given immunity from prosecution under Afghan law.

President Hamid Karzai has long demanded that U.S. soldiers be answerable to Afghan law, but the United States insists that its soldiers accused of crimes in Afghanistan are tried in America.

Underscoring the likely difficulty of the talks, U.S. military prosecutors are seeking the death penalty for Army Staff Sergeant Robert Bales, accused of killing 16 Afghan villagers in a March massacre. The Afghan government wants Bales to be publicly tried in Afghanistan.

"Afghanistan wants a strategic pact with U.S. but will seriously consider the red lines," said Aimal Faizi, Karzai's chief spokesman.

"The negotiation between the two countries are due to start today and the most important issue for Afghanistan is its national sovereignty and national interest," Faizi told Reuters.

The U.S. delegation is headed by James B. Warlick, the deputy special representative for Afghanistan and Pakistan. Eklil Hakimi, Afghanistan's ambassador to the United States, leads the Afghan team in Kabul.

This year, the two countries signed a strategic agreements which provides a framework for a post-2014 U.S. role in Afghanistan, including aid assistance and governance, but not troop numbers.