N'DJAMENA, Chad - Soldiers from Chad and Niger launched the largest international push to defeat Nigeria's Islamic extremists, whose war has spilled over into neighboring countries, officials and witnesses said Monday. Chad's president has warned that the leader of Boko Haram must surrender or be killed.

At least 200 vehicles full of soldiers were spotted by residents crossing from Niger into Nigeria. Loud detonations were soon heard, signaling heavy combat with Boko Haram, said Adam Boukarna, a resident of the border town of Bosso, Niger.

The push marks a sharp escalation by African nations against Boko Haram nearly six years after the group began its insurrection. At an African Union summit in Addis Ababa, Ethiopia, on Jan. 31, African leaders agreed to send 7,500 troops to fight Boko Haram. Later, neighboring countries agreed to increase the force to 8,750. U.N. Secretary General Ban Ki Moon has said he supports the AU's move.

The new offensive includes troops from Niger for the first time, in addition to Chadian forces that were already carrying out missions in Nigeria, Chadian Brig. Gen. Zakaria Ngobongue said Monday. He described extremism as a "cancer" in the region that could not be defeated by any one country alone.

"They are bandits and criminals who have nothing to do with religion," Ngobongue said, speaking to reporters after the closing ceremony for Flintlock, an annual training exercise in counter-insurgency tactics involving 20 countries.

U.S. Army Gen. David M. Rodriguez, commander of the U.S. Africa Command, which sponsored the exercise, noted here on Monday that the Islamic insurgents have been operating not very far from this dusty capital: The group has carried out attacks this year as close as about 90 miles away. N'Djamena is about 18 miles from the Nigerian border.

"We find this year's exercise both unique and relevant because as you know ... we are not far from the immediate threat of Boko Haram," the American general, who had earlier commanded U.S. military forces in Afghanistan, told reporters.

Nigeria and its allies will defeat Boko Haram, said Mike Omeri, a Nigerian government spokesman.