Members of an Afghan grand assembly debating a crucial security pact with the United States voiced concern Saturday at President Hamid Karzai's insistence it will not be signed until after April's election to choose his successor.

Kabul and Washington have agreed a joint draft of the Bilateral Security Agreement (BSA), governing the presence of US troops in Afghanistan after 2014, which is being debated by the "loya jirga", a gathering of about 2,500 chieftains, tribal elders and politicians.

But in his opening statement on Thursday, Karzai told the meeting that the painstakingly negotiated deal would not be signed until after next year's presidential election -- sparking a strong response from Washington, which wants it signed by the end of this year.

US officials warned that failure to sign the pact -- which governs the conditions of any post-war American counter-terrorism and training mission in Afghanistan -- could jeopardise crucial aid to the war-torn country.

The White House has said it needs a swift decision to start planning the movement of any US troops, and warned that President Barack Obama had not yet decided whether or not to keep American forces in Afghanistan.

"Karzai doesn't have the right to say this, he is making a mistake," said Sebghatullah Mujadidi, the head of the jirga.

"They (the Americans) have accepted all the conditions set out by him and us. It would hurt Afghanistan if he does not accept it," he added.

Amir Mohammad Akhnudzada, a delegate from the volatile southern Helmand province, said: "I think President Karzai should respect the decision of the Afghan elders, and all the delegates want this Bilateral Security Agreement signed as soon as possible."

Earlier, Karzai's spokesman Aimal Faizy told AFP that the president would explain the reasons for his stance in his closing speech to the jirga planned for Sunday.

On the penultimate day of the four-day jirga on Saturday, delegates debated the legal jurisdiction for US troops who remain in Afghanistan after 2014.

A draft text released by Kabul 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.

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.

"Undoubtedly, article 13 was a matter of concern to the people of Afghanistan. We discussed it thoroughly, and approved this article with some changes," said jirga member Abdul Qayom.

Anarkali Honaryar said: "This jirga was called mainly because of article 13 of the BSA. Everybody gives their suggestions and advice to the loya jirga."

Karzai on Thursday told delegates that the BSA would allow up to 15,000 foreign troops to stay in Afghanistan.

The jirga is to give its decision Sunday on the pact, the text of which was only agreed after months of tortuous negotiations with Washington.

The jirga is widely expected to approve it, though possibly with recommendations or conditions, after which it must also be approved by the Afghan parliament before it can be signed into effect.

Supporters of the deal say it is vital for post-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.

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