The United States Thursday warned Afghanistan to sign a new security pact as soon as possible, as top officials hinted that prolonged delays could mean no post-2014 US troop presence.

Washington's latest run-in with President Hamid Karzai was set off by the Afghan leader's statement that the painstakingly negotiated pact should not be signed until after his country's next election in April.

But US officials bristled, saying the Bilateral Security Agreement (BSA), which governs conditions of any post-war American counter terrorism and training mission in Afghanistan, must be signed by the end of the year.

"We must move forward as quickly as possible to sign the agreement," said State Department spokeswoman Jen Psaki.

The White House said it needed a swift decision from Karzai to start planning the footprint of any US forces, and trying to exert leverage, said Obama had not yet decided on whether to keep US forces in Afghanistan.

"Failure to get this approved and signed by the end of the year would prevent the United States and our allies from being able to plan for a post-2014 presence," White House spokesman Josh Earnest said.

"We have not yet determined whether or not a troop presence will continue in Afghanistan," Earnest said.

Other senior officials, speaking on condition of anonymity, were more blunt, warning that it was not practical for the BSA to await the signature of the next Afghan president.

They said that if there was no BSA in force, there would be no post-2014 US troop garrison in Afghanistan after NATO combat troops leave.

Karzai said the BSA would allow up to 15,000 foreign troops to stay in the war-torn country.

He also warned the pact currently under consideration by a loya jirga, a meeting of tribal chieftains, could 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 his two terms. A credible election is seen as crucial to the country's future stability.

The BSA governs the terms and legal status of US troops who might stay behind in Afghanistan, and the draft, which must also be approved by the Afghan parliament, emerged after tortuous negotiations.

Obama stepped in this week with a letter to confirm a deal on the vexed question of US forces raiding Afghan homes.

American officials quickly knocked down a flurry of media reports however suggesting that Karzai would get an apology for the price borne by Afghan civilians during the 12-year war.

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.

Supporters of the deal say it is vital for 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.

'No trust'

Karzai also gave a brutal 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 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.

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.

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

But Earnest said that did not mean that US troops would be in the war-torn nation until then.

"It wouldn't take that long," Earnest said.