Men on motorbikes ambushed a convoy of U.N. peacekeepers in northern Mali on Friday, U.N. officials said, killing nine in the deadliest attack yet on the force.

It was the latest in a string of deadly attacks on the peacekeeping force tasked with bringing stability to the West African country following a coup and jihadist offensive, in what has become one of bloodiest U.N. missions.

The convoy of troops from Niger included a fuel truck and may have been specifically targeted because of that, said U.N. spokesman Stephane Dujarric.

"Our understanding is that they were targeted and they were targeting a convoy that included a fuel truck, knowing full well that a fuel truck, I think, would cause an even greater number of casualties, which I think adds to the horrendous nature of the crime," he said.

The U.N. said in a statement that Secretary-General Ban Ki-moon was shocked and outraged by the attack.

Air support was immediately deployed to secure the area where the attack took place 9 miles east of Indelimane in the northern Gao region, the peacekeeping force said in a statement.

The attackers were carrying heavy arms, according to Olivier Salgado, a spokesman for the force, known as MINUSMA.

"This is the deadliest attack that MINUSMA has suffered since the beginning of the mission," he said. "The toll is rather serious."

Although there were initially thought to be troops wounded in the attack, a U.N. peacekeeping official said there were no injuries. He spoke on condition of anonymity because he was not authorized to speak publicly on the matter.

The official said the ambush raises to 30 the death toll for the peacekeeping operation, which was established by the U.N. Security Council in April 2013.

Last month, a roadside bomb killed five peacekeepers and wounded several other Chadian troops near the embattled city of Kidal. Mines killed five other peacekeepers last month.

Northern Mali fell under control of Tuareg separatists and then al Qaeda-linked Islamic extremists following a military coup in 2012. A French-led intervention last year scattered the extremists, but some remain active and there have been continued bursts of violence.

U.N. troops are now trying to stabilize the north, and peace talks have begun between the Malian government and Tuaregs. In late June, the force comprised of 11,200 military personnel and 1,440 international police.

But as French troops have drawn down, the situation has become "intolerable," U.N. peacekeeping chief Herve Ladsous told reporters on Saturday.