Gunmen ambushed a convoy of traders travelling to a market under military escort and killed at least 20 people in Nigeria's northwestern Zamfara state, a witness and a military source said on Monday.

Local resident Musa Danladi told Reuters the attack occurred around 2 pm on Saturday with the convoy en route to Nasarawa market in Bukkuyum district from Gwashi.

Danladi, who helped retrieve the bodies, said four soldiers, three policemen and a Nigerian civil defence corps officer were among those killed, along with 12 civilians.

Northwestern Nigeria has seen a surge in attacks by heavily armed gangs, known as bandits, who raid villages, abduct residents for ransom and rustle livestock.

A military source confirmed the attack and said 19 people were also wounded.

Military and police authorities did not immediately respond to requests for comment.

Separately, the military said an air strike in northeastern Borno state killed more than 40 Islamic State West Africa Province (ISWAP) fighters, including a senior commander, on October 10.

Military spokesperson Captain Mohammed Goni said the strike targeted an ISWAP enclave in Badu village after intelligence indicated the fighters were planning coordinated attacks on troops.

Goni said the fighters belonged to ISWAP's Murabitun unit, a mobile force involved in attacks and patrols, and that the strike had significantly weakened the group's operational capabilities in the region.