The fighting was in the Sambisa Forest Reserve, just south of Maiduguri, the capital of Borno state, which soldiers previously raided on the hunt for fighters belonging to the extremist network known as Boko Haram.
Meanwhile, gunmen launched an assault on the hometown of one of Nigeria's former military rulers hundreds of miles (kilometres) away, attacking a police station and banks.
Soldiers started the attack on Sambisa Forest Reserve yesterday, having previously converged in the area in advance of President Goodluck Jonathan's state of emergency decree affecting three states in the nation's northeast, a security official said.
"We are not going to leave the forest until it's over," the official said, referring to the emergency rule.
The official spoke to The Associated Press on condition of anonymity as he was not authorised to speak publicly about the ongoing military operation. Brigadier-General Chris Olukolade, a military spokesman based in Nigeria's capital, Abuja, could not be immediately reached for comment today.
In a related development, mobile phone service returned this morning to parts of northeast Nigeria after being cut yesterday. The security official told the AP that the service cut came on the orders of Nigeria's government and security forces as soldiers moved into the northeast to begin operations. The official said service likely would be shut off again.