Afghan President Hamid Karzai gave his backing Thursday to a proposed security pact with the United States that will see up to 15,000 foreign troops stay in the war-torn country.

But he said it would not be signed until after next year's election.

A grand assembly of tribal chieftains, community elders and politicians began four days of debate on the Bilateral Security Agreement (BSA), which will shape Washington's future military presence in Afghanistan.

Hours before the meeting, US Secretary of State John Kerry said the two sides had finally agreed the text of the pact after months of difficult negotiations.

A letter to Karzai from US President Barack Obama confirmed an agreement announced by Afghan officials on Tuesday over the vexed question of US forces raiding Afghan homes.

The letter released by Karzai's office said US forces would not enter Afghan homes for military operations "except under extraordinary circumstances involving urgent risk to life and limb of US nationals".

The issue is a sensitive one in Afghanistan and had for a time appeared to pose a serious threat to the deal.

If the "loya jirga" assembly approves the BSA, it must then be passed by the Afghan parliament.

It has been touted as vital to the country's future after 2014, when the bulk of NATO's 75,000 troops will pull out. The Taliban insurgency this year has reached levels of violence not seen since 2010, according to the United Nations.

Karzai urged the 2,500 delegates to consider Afghanistan's "future prosperity" as they made their decision, saying the deal gave the country a chance to move on after more than 30 years of war.

"This agreement provides us a transitional period to reach stability in the next ten years ahead of us," he said.

He signalled yet another delay to the pact, which Washington had wanted completed by the end of October, saying it would only be signed "when our elections are conducted, correctly and with dignity".

Afghanistan goes to the polls on April 5 to elect a successor to Karzai, who must step down after serving two terms. A credible election is seen as crucial to the country's future stability.

Karzai told delegates Afghanistan needed Washington's cooperation in ensuring a clean, fair ballot.

'No trust'

The deal will see 10,000-15,000 foreign troops remain in Afghanistan after NATO pulls out the bulk of its forces by the end of 2014, Karzai said.

But he stressed that not all would be American, saying there would be troops from other NATO countries and "some other Muslim nations".

And he gave a frank assessment of his often thorny relationship with Washington, his principal foreign backer.

"America does not trust me and I do not trust them. I have had struggles with them and they have spread propaganda against me," he said.

The Taliban have condemned the jirga as an American plot and threatened to target its delegates if they approve the deal.

Last week a suicide bomber detonated a car bomb near the jirga area, killing 12 people.

A draft text released by Kabul late Wednesday appeared to show Karzai had bowed to a US demand that American troops would not be tried in local courts if they are accused of crimes -- an issue that became a major hurdle in the negotiations.

A similar security deal between the United States and Iraq collapsed in 2011 over the issue of whether American troops would be answerable to local courts, leading Washington to pull its forces out.

But the text, published on the Afghan foreign ministry website, said Kabul had agreed that the United States should have "the exclusive right to exercise jurisdiction" over its forces in Afghanistan.

"Afghanistan authorises the United States to hold trial(s) in such cases, or take other disciplinary action, as appropriate, in the territory of Afghanistan," it said.

It adds the deal will remain in force "until the end of 2024 and beyond", unless either side ends it.

Kerry said the remaining forces would have "a very limited role, it is entirely train, equip, and assist. There is no combat role for the United States forces."

The draft deal also said that while US military operations against Al-Qaeda may be "appropriate" in the fight against terrorism, the two sides would cooperate closely to protect US and Afghan interests "without unilateral US military counter-terrorism operations".