Nine U.N. peacekeepers were killed in an ambush Friday in Mali, a U.N. spokesman said.

Their convoy was attacked near Menaka, in the country's eastern Gao region, by heavily armed men on motorcycles, said Olivier Salgado, a spokesman for the U.N. Stabilization Mission in Mali.

The nine dead were from Niger. The United Nations does not know who is responsible, Salgado said.

The attack was the deadliest yet of several against the forces in the country, the U.N. mission said on its Facebook page.

Air assets were immediately deployed with the aim of making the area safe and helping the peacekeepers, the statement said.

Arnauld Akodjenou, deputy special representative of the secretary-general in the U.N. mission, called for an end to violence.

He said he was horrified by "this cowardly and hateful act of terrorism" and the loss of more lives in pursuit of peace in Mali, adding that these crimes must not go unpunished.

In a statement, U.N. Secretary-General Ban Ki-moon said such attacks constitute a violation of international law, and called on armed groups operating in the area of promises they have made to cooperate with the United Nations.

The latest attack comes a little more than two weeks after five U.N. peacekeepers from Chad were killed and three others injured when a U.N. vehicle struck a homemade bomb in northern Mali.

The September 18 attack took place on a road between Aguelhok and Tessalit, the U.N. mission said.

Other attacks have also killed or injured U.N. peacekeepers in the Kidal region.

The U.N. mission is there to guard against militant Islamists who in early 2013 threatened to move on the West African nation's capital, Bamako.

More than 8,000 military personnel and nearly 1,000 police were deployed under the peacekeeping mission as of the end of August. It was set up in April 2013 following a U.N. Security Council resolution.