Chadian soldiers have killed 20 Boko Haram fighters and freed 12 civilians, including nine children, kidnapped in the Lake Chad area in which several countries' borders meet, the government said Friday.

The extremist group, which originated in Nigeria in 2009, has established bases on islets dotting Lake Chad – a vast, swampy expanse on the border between Nigeria, Chad, Niger and Cameroon.

It has stepped up attacks in the area in recent months.

On September 17, Boko Haram fighters raided a village in the restive zone and kidnapped the civilians, Communications Minister and government spokesman Cherif Mahamat Zene told Agence France-Presse (AFP).

The army pursued the raiders and attacked them Thursday in Barkalam, near the Nigerian border, he said, "killing 15" and "freeing 12 civilians."

A little later, there was another encounter at Bilabrim in which five Boko Haram fighters were killed and two Chadian soldiers were wounded.

The Chadian army launched an offensive against Boko Haram in April after the deaths of some 100 soldiers in an attack by the group on one of its bases.

President Idriss Deby then claimed to have pushed the Boko Haram fighters out. But attacks have continued despite the military operation.

In Chad's Lake Province, more than 360,000 people have fled their homes to avoid attacks and also flooding, according to the International Organization for Migration (IOM).

The president admitted in early August that "Boko Haram would still do a lot of damage" in Chad.

Boko Haram's insurgency has killed more than 36,000 people and displaced more than two million from their homes.

The violence has since spread to Niger, Chad and Cameroon.