KABUL: Afghanistan and United States have solved a key sticking point in a crucial security pact just two days before it was due to be voted on by Afghan tribal and political leaders, an Afghan official said Tuesday.

Aimal Faizi, President Hamid Karzai's spokesman, told reporters in Kabul that the deal would allow US troops to enter Afghan homes once Nato forces withdraw in 2014 but only in “extraordinary circumstances” where there was an urgent risk to life.

The compromise appeared to end an impasse which had threatened to derail the agreement.

Faizi said President Karzai and Secretary of State John Kerry spoke by phone Tuesday during final negotiations for the bilateral security treaty (BSA) which will shape Washington's future military presence in the war-scarred nation.

However officials in Washington said there was still some way to go before reaching a final agreement on the pact.

“We're not there yet. There are still some final issues we are working through,” said State Department spokeswoman Jen Psaki.

She confirmed that Kerry had spoken with Karzai on Tuesday morning, adding “obviously, there has been some progress made to resolve outstanding issues”.

But she refused to go into any details.

Even if a final agreement is reached, Afghanistan has insisted that the BSA must be approved by a mass gathering of tribal chieftains and politicians.

The four-day grand assembly, known as a “loya jirga” in Pashto, is set to begin on Thursday in Kabul.

Key obstacles to finalising the pact have included the question of legal immunity for those US troops who remain and whether they will be able to search the homes of Afghan citizens.

Faizi said both sides had now agreed to the “extraordinary exception” clause on house searches.

He also said President Barack Obama would write to Karzai and the Afghan people assuring them that US troops would not “misuse” the searches and that “those extraordinary circumstances would be strictly defined”.

He added that Karzai would expect the letter to acknowledge the fact that Afghan people have suffered in the last decade.

“What is important for the Afghan government is the US acknowledging the fact that there have been mistakes in the war on terror, that in the conduct of military operations in Afghanistan mistakes were made and that the people of Afghanistan have suffered in the last decade,” he said.

Officials at the White House Tuesday said they would not comment on any presidential correspondence.

And a senior State Department official said later Tuesday that the request for a letter had come from Karzai during the phone call with Kerry.

“Secretary Kerry indicated that we want to continue working together to find a path forward, and that we will consider his request for reassurances including the option of a letter from the administration stating our position,” the official said.

Karzai had also invited Kerry to attend the loya jirga, but the US top diplomat said “it would not be possible for him to attend this week,” the official said.

The BSA will determine how many US soldiers stay in Afghanistan when most of the Nato's troops deployed in the country since 2001 – currently numbering 75,000 –leave at the end of 2014.

The Taliban have branded the loya jirga meeting a US-designed plot, vowing to pursue and punish its delegates as traitors if they approve the BSA.

Highlighting the ongoing security challenges facing Afghanistan, a Taliban suicide bomber on Saturday struck close to the venue where the meeting is set to be held killing a dozen people, most of them civilians.

The issue of US troop immunity sank a similar deal in Iraq in 2011, leading the Americans to pull out completely.

The country is now in the grip of some of its worst sectarian violence since 2008.