A deal to allow some US troops to stay in Afghanistan to fight al-Qaida is teetering on the edge of collapse after Afghan President Hamid Karzai said he was prepared to walk away from negotiations.

The US has pushed for a bilateral security pact to be signed by the end of this month so the US-led Nato force can schedule its withdrawal of 87,000 combat troops by the end of next year.

But Mr Karzai said he refused to be rushed into signing the deal and would first seek approval from a grand assembly in a month's time.

"The agreement has to suit Afghanistan's interests and purposes.

"If it doesn't suit us and if it doesn't suit them, then we will go separate ways," Mr Karzai said in Kabul.

According to the Afghan government, talks ground to a halt over US demands for the right to conduct unilateral operations after 2014 and on how the US would pledge to protect Afghanistan.

US Defence Secretary Chuck Hagel described the deal as "critically important" last week.

Mr Karzai is keen to resolve the disagreement before he steps down next year, but his stance on the agreement matches his incendiary accusation that the Nato war effort had been useless.

"On the security front, the entire Nato exercise was one that caused Afghanistan a lot of suffering, a lot of loss of life, and no gains," he said.