Abuja, Sep 22 (IANS): At least 13 insurgents were killed in an exchange of fire with Nigerian forces after a local market was attacked in the northeastern state of Borno, a top military source told Xinhua Monday.

The Nigerian troops swung into action after the attackers, suspected to be members of the Boko Haram group, hit an overcrowded market in Mainok, 56 km from Maiduguri, Borno's state capital Friday, the security source who asked to to be anonymous said via telephone.

Earlier reports said more than 20 civilians died when the militants fired grenades in the market square. But the Nigerian troops, having received a distress call, promptly arrived at the scene and engaged the insurgents in a shootout that led to the death of 13 of them, the security source said, adding some of the attackers fled with bullet wounds.

A commuter bus driver, Ba'ana Gunda, who plies the market route, said some of the corpses aere still littered the area as of Monday morning.

"But we noticed that, this (Monday) afternoon, many of the bodies had been moved. It was a gory sight as we drove past the area early Monday," he added.

Reports from Mainok also indicated that survivors wore a forlorn look, counting their losses and recounting Friday's bitter experience. Many of the traders claimed their shops were ransacked during the attack.

At least 34 deaths were recorded in the attack, although officials were yet to mention casualty figures as of Monday.

The Boko Haram Islamist sect has proved to be a major security threat in Nigeria, Africa's most populous country, in the last five years.