Kabul is Nato's first major assignment outside Europe

Nato Secretary-General Lord Robertson made the announcement during a visit to Kabul on Friday.

Nato currently heads the 5,000 strong International Security Assistance Force (Isaf) which is confined to Kabul.

Afghan President Hamid Karzai and UN Secretary-General Kofi Annan have been urging an expansion of Isaf's mandate amid a surge in violence blamed on suspected Taleban fighters.

Many of the attacks have targeted aid workers, raising a question mark over the future of the rehabilitation and reconstruction process.

Troop visit

In Kabul, the Nato secretary-general said the organisation was reviewing its plans to expand the mission.

"We've listened very carefully to the voices of those who've said that bringing security to Kabul alone is not enough," Lord Robertson said.

He visited Isaf troops stationed in Kabul as well as senior government officials during his one-day visit.

Lord Robertson says Nato will stay as long as necessary

Earlier he said Nato was determined to contribute for as long as necessary to Afghanistan's peace and security.

"Security must be established throughout the country, and Nato is now examining how best to contribute to this," he said.

Among the options being considered is for Nato to take over the provincial reconstruction teams (PRTs) which are mobile civil-military units involving American and British troops.

But any decision to expand Isaf's mandate would have to be approved by the UN security council.

Afghanistan is Nato's first assignment outside Europe, but some of its member countries are said to be reluctant to commit more troops to Afghanistan.

Aid worker killed

A number of humanitarian workers have been attacked in the south and east of the country, once the stronghold of the Taleban.

An Afghan aid worker was killed on Wednesday in an ambush carried out by suspected members of the Taleban.

It was the second fatal attack on aid workers in south-eastern Afghanistan this month.

Armed men opened fire on the aid worker's vehicle as it was travelling in the province of Helmand, north-west of Kandahar.

Four Afghan aid workers employed by a Danish charity were killed earlier this month in the province of Ghazni when armed men stopped their car.

In August two aid workers from the Afghan Red Crescent were killed in an ambush south-west of Kabul.