David Cameron expresses his "deep concern" about recent rebel advances in Mali as French forces carry out further air strikes against Islamist rebels on Saturday.
France's defence minister says hundreds of French troops are involved in an operation that destroyed a command centre of Islamic rebels in Mali.
Jean-Yves Le Drian said that a French helicopter pilot died of his wounds in the operation, which involved air strikes on three rebel targets overnight.
The French president authorised the operation to support Mali's government after the Islamists launched an offensive outside the territory they had previously captured.
Le Drian said France was compelled to act quickly to stop the Islamist offensive, which he said could allow "a terrorist state at the doorstep of France and Europe."
Niger has said it will send 500 soldiers to join the military campaign in Mali, Foreign Minister Mohamed Bazoum said.
"We are sending a battalion of around 500 individuals under the ECOWAS plan," Bazoum told Reuters by telephone, adding that he could not say exactly when the troops would arrive.
Read more from International Editor Lindsey Hilsum: Will anyone stop the Islamists in Mali?
Cameron 'deeply concerned'
David Cameron has expressed his "deep concern" about the situation in Mali. The prime minister said the events show that it is "crucial" for the international community to co-operate in tackling terrorism in Africa.
"I am deeply concerned about the recent rebel advances in Mali, which extend the reach of terrorist groups and threaten the stability of the country and the wider region," Mr Cameron said.
"I welcome the military assistance France has provided to the Malian government, at their request, to halt this advance.
"These developments show the need to make urgent progress in implementing UN Security Council resolutions on Mali, and ensure that military intervention is reinforced by an inclusive political process leading to elections and a return to full civilian rule.
Operation to last 'as long as needed'
France has told the UN Security Council that its military operations would last as long as needed.
"This operation, which takes place within the framework of international law, will last as long as necessary," France's UN ambassador, Gerard Araud, wrote in a letter to the Security Council obtained by Reuters.
"The evolution of the situation justifies the acceleration of the implementation of resolution 2085," he added, referring to a resolution adopted last month that called for deployment of an African-led force in support of the Malian army.
UN officials have said the African-led force would not be deployed before September.