At least 26 members of the Nigerian security forces were killed and eight wounded in an ambush by gunmen in central Nigeria late Sunday, two military sources told AFP.
Additionally, an air force spokesman said a helicopter rescuing the wounded crashed on Monday morning in the area, where the army is fighting criminal groups, without specifying whether the crew and passengers had survived.
The two military officers asked not to be identified because they were not authorised to speak on the incident while military authorities were not available for comment.
"We lost 23 soldiers, including three officers, and three Civilians JTF (vigilantes) in the encounter while eight soldiers were injured," said the first source, following "a serious fight" along the Zungeru-Tegina highway.
A second officer gave the same toll and said the bandits also suffered "heavy casualties". He also said that communication had been lost with an air force helicopter dispatched to evacuate the casualties, with 11 of the dead and seven of the injured aboard.
He said the helicopter was carrying 11 of the dead and seven of the wounded. He added that the aircraft had crashed because of gunfire from "bandits".
A Nigerian air force spokesman confirmed that its Mi-171 helicopter while on a "casualty evacuation mission" crashed on Monday after take-off from Zungeru.
"The aircraft had departed Zungeru Primary School enroute for Kaduna but was later discovered to have crashed near Chukuba Village in Shiroro Local Government Area of Niger State," spokesman Edward Gabkwet said in a statement.
He said efforts were under way to rescue those aboard and that preliminary investigations had been opened into the cause of the crash.
Barely a week goes by in Africa's most populous nation without attacks or kidnappings by criminals known as "bandits" in the northwest and centre of the country.
The gangs, who have been notorious for mass school abductions, maintain camps in a vast forest straddling the states of Niger, Kaduna, Zamfara and Katsina.
Northwest and central Nigeria have for years been terrorised by bandits who raid remote villages where they kill and abduct residents for ransom, as well as burn homes after looting them.
Impunity as well as insufficient security and wider government presence has allowed the violence to fester, experts say.