Nigeria's air force on Thursday bombed Boko Haram strongholds in northeast Borno state, killing "a large number of terrorists", as a four-nation offensive against the insurgents presses on, a military statement said.

The targets of the bombardment included the town of Gwoza, where Boko Haram leader Abubakar Shekau first proclaimed the existence of a caliphate inside in Nigeria in August, and the notorious Sambisa Forest area, where the rebels have had camps for years.

Boko Haram militants suffered heavy casualties when Chadian troops pushed into Nigeria this week, residents who fled the fighting told Agence France Presse on Thursday.

Chad's army said on Tuesday evening that they had seized control of the town of Dikwa, which is about 50 kilometers (31 miles) southwest of the Nigerian border town of Gamboru.

Four African armies, from Nigeria, Cameroon, Chad and Niger, are currently directly engaged in the fight against the Nigeria-based insurgent group Boko Haram.

Regional forces have gone into action while awaiting the formation of a planned 8,700-strong, five-country force, whose make-up remains to be determined.

Nigeria on Wednesday claimed to have killed more than 300 Boko Haram militants in a counter-offensive, as Chad pushed into rebel-held areas but an air strike left at least 20 civilians dead.

Boko Haram meanwhile vowed to make voting impossible in Nigeria's upcoming presidential and parliamentary elections after a brace of deadly suicide attacks in the country's violence-wracked northeast.

Indonesia's foreign ministry told Australia Wednesday that nobody responds well to threats as tensions mount over the looming execution of two Australian drug smugglers.

As diplomatic efforts intensify to save Andrew Chan and Myuran Sukumaran, Prime Minister Tony Abbott asked Indonesia to remember the one billion dollars in Australian aid in the aftermath of the 2004 tsunami that killed 170,000 Indonesians.

Islamist group Boko Haram has vowed to make voting impossible in Nigeria's upcoming election after a spate of deadly attacks in the country's violence-wracked northeast.

The group's leader Abubakar Shekau said in a new video posted on Twitter late on Tuesday evening that his fighters would disrupt polling in Africa's most populous nation scheduled for March 28.

Eleven people were killed in a suicide bomb attack on a checkpoint near the town of Biu in northeast Nigeria on Tuesday, a vigilante and a resident said.

Gana Sheriff, the vigilante, said the attack happened at about 1:00 pm (1200 GMT) at Yamarkumi village, four kilometers (2.5 miles) from Biu in Borno state.

Hundreds of Boko Haram militants stormed the town of Askira Uba in northeast Nigeria, burning homes and public buildings, as attacks continued despite a regional campaign against the Islamist rebels, witnesses said Tuesday.

Residents able to flee the town in Borno state left in droves, with only the sick and elderly remaining behind through the insurgents' rampage.

Niger's President Mamahadou Issoufou on Tuesday told thousands of people protesting at deadly raids by Boko Haram that "Niger will be the tomb" of the Islamists from neighboring Nigeria.

"Nobody attacks Niger without punishment and Boko Haram learned that to its cost last February 6," Issoufou told a crowd after a rally in the capital Niamey. "That day, our defense and security forces crushed Boko Haram."

Cameroon's army says it has killed 86 Boko Haram militants and detained 1,000 people suspected of links to the Islamist group, as central African leaders held talks on how to combat its bloody insurgency.

Five Cameroonian soldiers were also killed during the clashes in the Waza region near the border with Nigeria, defense ministry spokesman Didier Badjeck said Monday.