The UN Security Council on Thursday voted a final mandate for the US-led international military force in Afghanistan before it withdraws at the end of 2014.

A resolution unanimously backed by the 15-nation council authorized the International Security Assistance Force (ISAF) to "take all necessary measures" to carry out its duties until December 31, 2014.

The United States and other western nations -- who have been in Afghanistan since just after the September 11, 2001 attacks -- say they will hand over all security duties to Afghan forces by the end of next year.

While western countries remain nervous about Afghanistan's future, they have already started to withdraw troops while still battling the Taliban.

There are still about 87,000 troops in ISAF. About 60,000 are US forces but 49 countries remain contributors.

ISAF has had rocky relations with President Hamid Karzai's government, but troop numbers are steadily falling.

A security deal to allow some US troops to stay in Afghanistan to fight Al-Qaeda appeared at risk of collapse after Karzai threatened to walk away from negotiations.

According to the Afghan government, talks ground to a halt over US demands for the right to conduct unilateral military operations after 2014.

The United States has pushed for a bilateral security pact to be signed by the end of this month so the ISAF withdrawal can remain on schedule.

Britain on Thursday started its last major deployment to Afghanistan, with some 6,000 troops to be sent to Helmand in southern Afghanistan over the next six weeks, the government announced. The 8,000 British troops in Afghanistan will be cut to 5,200 by the end of this year.

The UN Security Council did not specifically say there would be no new mandate. But its resolution, drafted by Australia, said ISAF's "final report in December 2014 should be a comprehensive one."

The resolution highlighted international support for Afghanistan after 2014 but also Afghanistan's promises at international conferences to improve its electoral organization ahead of a presidential vote in April.

It also stressed "the need for further efforts by the Aghan government to fight corruption, promote transparency and increase its accountability."