Kabul is Nato's first major assignment outside Europe
The United Nations Security Council has unanimously adopted a resolution enabling international peacekeepers to operate beyond the Afghan capital Kabul.
The measure, which passed 15-0 with no abstentions, allows the Nato-led force to deploy in a number of provincial towns.
It wants to start by sending German troops to the northern city of Kunduz.
The vote has been warmly welcomed by the Afghan administration.
Afghan President Hamid Karzai has long demanded that the force's mandate be expanded, in an effort to reassert his control beyond Kabul.
Remnants of the former Taleban regime carry out almost daily attacks in some provincial areas.
Nato took command of the 5,300-strong International Stabilisation Force in Afghanistan (Isaf) in August.
German and Canadian troops make up the bulk of the force - which is separate from the US-led operation against Taleban remnants in southern and eastern Afghanistan.
The BBC's Crispin Thorold in Kabul says the security situation in the provinces is impeding development projects.
A spokesman for the Afghan foreign ministry said he is confident that an expansion of peacekeeping operations outside Kabul will yield positive results.
More troops needed
The first new troops will come from Germany, which has said it wants to send up to 450 soldiers to Kunduz to form a Provincial Reconstruction Team.
In the longer term, the alliance wants to create "islands of security" in other urban centres.
It is not clear where the other extra troops would come from.
"This resolution helps pave the way for the increased security in Afghanistan on which everything else is dependent," said US Ambassador to the UN John Negroponte.
However, critics say this is too little, too late, our correspondent reports.
Alliance planners last month proposed various options for expanding Isaf, suggesting additional forces of between 2,000 and 10,000 would be needed.
At present many areas of the country are off-limits to aid agencies because of the threat of violence.
They have called on the international community to address threats from extremists and warlords and say the country's hopes of reconstruction are growing increasingly fragile.