DAKAR, Senegal - A mortar attack on a U.N. base in northern Mali early Saturday killed at least three people, including two U.N. peacekeepers and a contractor, said the mission spokesman. The attack also injured 20 people, and four of them are in serious condition, said Olivier Salgado, spokesman for the mission.

No one claimed responsibility for the attack in the country's restless north but Islamic extremists are suspected. In 2013, the French pushed Islamic militants out of Mali's northern cities and towns although jihadists continue to carry out attacks on U.N. peacekeepers.

"These attacks will not impede the determination of the United Nations to support the Malian people and the peace process," said Mongi Hamdi, the head of the U.N. mission and the special representative for the secretary-general in Mali. "I express my solidarity and salute the brave men and women serving [the U.N. mission] throughout the country."

Northern Mali has been unstable since it fell to Tuareg separatists and Islamic extremists after a military coup in 2012. Despite the presence of French troops and the U.N. force, the extremists have continued attacks, even extending them farther south, including an assault on a Bamako restaurant/nightclub popular with foreigners in March and a Nov. 20 attack on the Radisson Blu hotel in the capital.