Clashes between Chadian troops and Boko Haram fighters during a raid on a northeastern Nigerian town have killed 200 fighters and nine soldiers, Chad's military says.

"We regret nine dead and 21 wounded. On the enemy side: more than 200 deaths," a statement cited by the AFP news agency said in reference to Tuesday's clashes.

It added that the toll could rise since clean-up operations in the town of Gamboru on Nigeria-Chad border.

Earlier, Cameroon soldiers clashed with Boko Haram fighters in the border town of Fotokol as the fighters fled an offensive by Chad's army, security sources said.

The fighters "entered this morning," a Cameroonian security source said, after Chadian troops recaptured Gamboru just across the border.

"Fighting between them and our soldiers is really intense."

In a deserted Gamboru, Chadian forces carried out clean-up operations after entering the town on Tuesday and retaking it from Boko Haram, which seized control months ago.

"When the Chadians entered Gamboru, the Boko Haram members who were in the town and some villages fled to meet up this morning in Fotokol," the security source said.

Regional military efforts

While a bridge separates Fotokol from Gamboru, it is possible to also cross over from surrounding villages.

Chadian jet fighters had also bombed Gamboru before its troops entered.

The regional military efforts have highlighted the failure of Nigeria's army to stop the six-year Boko Haram insurgency that has killed thousands.

Nigeria's military said on Tuesday the country's sovereignty was not compromised despite the presence of Chadian ground troops and claimed to be "driving the present onslaught".

The offensive comes at a crucial moment, with Nigeria's presidential and parliamentary elections set for February 14.

Boko Haram opposes Western education and wants to impose Islamic law in all 36 states of Nigeria, which is roughly equally divided between a predominantly Muslim north and a mainly Christian south.

The group operates mainly in the northeastern Nigerian states of Adamawa, Borno and Yobe, where the government has declared a state of emergency since May 2013.