BAMAKO, Mali — A suicide truck bomb killed at least 60 people, including army personnel, in northern Mali on Wednesday as the country struggles to implement a peace deal crafted after Islamist groups, some linked to al-Qaeda, took over key northern cities.

The group’s North African affiliate, al-Qaeda in the Islamic Maghreb (AQIM), asserted responsibility for the attack. Some Islamist extremist groups in the area have opposed a 2015 peace pact between Mali’s government and other militant factions. Among the dead were soldiers, members of pro-government forces and fighters from autonomous armed groups that have signed the peace accord. At least 100 people were injured.

The blast, one of the worst attacks against the nation’s armed forces, happened in the northern city of Gao at a camp where 600 men from two pro-government militias and the Malian army were based.

The unit had been trained by United Nations peacekeepers and French soldiers, but no U.N. or French troops were wounded or killed.

Just before 9 a.m., a suicide bomber in a truck full of explosives drove through the camp’s gate, said Radhia Achouri, a U.N. spokeswoman in Mali. “Whoever perpetrated [the] attack, it’s clear the target is the actual peace process,” she said.

A number of extremist groups, including AQIM, operate in northern Mali. Ethnic Tuareg rebels occupied Gao and other northern cities in 2012 and early 2013 and declared an independent ­Tuareg state. They were then absorbed into an al-Qaeda-allied insurgency.

After French troops intervened and scattered the radical Islamist forces, the United Nations tried to forge a peace agreement among the remaining armed groups, including the secular ­Tuareg separatists. The accord did not include the Islamists.

The agreement combined the groups under a single umbrella, called the Operational Coordination Mechanism, known by its French acronym, MOC.

The men who were killed Wednesday were MOC members who were expected to begin their first mixed patrol in the coming days — a key step in the implementation of the peace deal.

Islamist groups and other criminal elements who use northern Mali as a drug or arms-trafficking route oppose the creation of these mixed units and the peace deal itself, which they view as a threat to their territory.

Malian President Ibrahim Boubacar Keita declared three days of national mourning after the assault.

Extremist groups in northern Mali have staged devastating attacks over the past year on a U.N. base and the Gao airport, using suicide bombers and cars packed with explosives.

The amount of explosives involved has been staggering. In the attack on Gao’s airport in November, one of the trucks that did not detonate was found to contain more than 1,000 pounds of explosives, U.N. officials said.

The United Nations has more than 11,000 peacekeepers in Mali, which has become the world body’s most dangerous mission. More than 100 U.N. peacekeepers have been killed here since the mission began in 2013.

France’s ties to Mali run deep. The African state was a French colony for nearly 60 years, gaining its independence in 1960. Since 2012, France has been a major military presence in the region.

In January 2013, French President François Hollande insisted in a televised address that French troops would remain until the Islamist threat vanished. “At stake is the very existence of the Malian state,” he said.