Britain is to transport foreign troops and equipment to Mali but Downing Street says no troops will be used in a combat role.
The help was agreed in a phone call between David Cameron and France's President Francois Hollande, after the French sent in forces to support the Malian government.
Downing Street stressed that no UK troops would engage in combat operations in the country, but two transport planes are expected to be deployed within 24-48 hours.
"The prime minister spoke to President Hollande this evening (Saturday) to discuss the deteriorating situation in Mali and how the UK can support French military assistance provided to the Malian government to contain rebel and extremist groups in the north of the country," a spokeswoman said.
"The prime minister has agreed that the UK will provide logistical military assistance to help transport foreign troops and equipment quickly to Mali.
"We will not be deploying any British personnel in a combat role. They also agreed that the peacekeeping mission from west African countries needs to be strongly supported by countries in the region and deployed as quickly as possible.
"Both leaders agreed that the situation in Mali poses a real threat to international security given terrorist activity there."
Our mission is 'not over yet'
Mr Hollande made clear that France's aim in Mali was to support the west African troop deployment, which is also endorsed by the United Nations, the European Union and the United States.
Western countries in particular fear that Islamists could use Mali as a base for attacks on the west and expand the influence of al Qaeda-linked militants based in Yemen, Somalia and north Africa.
"We've already held back the progress of our adversaries and inflicted heavy losses on them," Mr Hollande said.
"Our mission is not over yet."