Unknown assailants attacked and withdrew inside a U.N. police base in the Malian city of Timbuktu on Friday morning, the United Nations said, while security sources said a Malian army checkpoint in the city had also come under fire.

The attackers drove up to the entrance of the U.N. camp at around 6:30 a.m. (1.30 a.m. EST) and then detonated their vehicle, said Olivier Salgado, spokesman for the U.N. peacekeeping mission in Mali, MINUSMA.

"An unknown number of assailants have withdrawn inside the camp. An operation is currently under way with Malian and MINUSMA forces," he said, adding that an injured police officer had been evacuated.

A military source in Timbuktu said the base was home to a Nigerian contingent. Salgado said it had been empty at the time of the attack, barring a few guards.

Simultaneously, a Malian army checkpoint in the Kabara neighborhood near the airport was attacked by gunmen. It was not immediately clear if there were any casualties.

Islamist militants, who briefly held the city of Timbuktu in 2012 until French forces drove them out, have stepped up attacks in Mali in recent months as part of a growing regional insurgency.

Last month, al Qaeda fighters seized a Swiss missionary living there and set conditions for her release.