The United Nations Security Council agreed Thursday to authorise a multinational peacekeeping force for the Afghan capital, Kabul, led by British troops with emergency backup from the United States should security deteriorate.

The 15-member council's decision is a rubber stamp on deployments already in the works for weeks.

But details still need to be worked out by several governments planning to send soldiers, including Germany, which needs parliamentary approval for military deployments abroad.

British soldiers started deploying in small numbers immediately, joining an unknown number of SAS special operations personnel and British intelligence agents already active in Afghanistan for months.

The UN force, known as ISAF the International Security Assistance Force will concentrate in the capital, the prize city in the eyes of rival Afghan armies who only recently inked a deal to co-operate in a "broad-based interim government under Prime Minister Hamid Karzai.

"The primary aim of the ISAF is to provide security in such a way that the interim authority feels confident to go about its work without security concerns," British Ambassador Sir Jeremy Greenstock told reporters.

Its mandate is valid for six months, and any extension will require support again from the Security Council.

The United States, having forced the regime change in Afghanistan by bombing the Taliban for two months and co-operating with Northern Alliance forces on the ground, will not contribute soldiers to the ISAF.

But US forces will play a key role, as crack troops in case security disintegrates.

After 20 years of war, Afghanistan is weary and ruined by war, but the political and military balance is fluid, and pockets of resistance to the new regime remain a threat.