Samantha Newport, Head of Communications, UNICEF Office for Coordination of Humanitarian Affairs (OCHA), confirmed the incident to News Agency of Nigeria (NAN) on Friday in Maidguri.
"I condemn the killings of members of staff of the United Nations (UN) and other humanitarian and aid workers in Rann by Boko Haram terrorists".
He said the United Nations is also concerned about other civilians who may have been injured or killed in the attack. "They fought the terrorists and we lost two men in the fight".
In the same vein, the European Union (EU) also condemned the attack.
The gunmen invaded their camp in Rann and during the attack, most of the aid workers fled into the military post which they called barrack to take refuge.
Mueller was on a 10-day Chad, mission to the Central African Republic (CAR) and Cameroon, where she saw see first-hand the devastating humanitarian consequences of ongoing violence.
The source claimed that her family had received information of her freedom from her abductors.
Impeccable sources close to the humanitarian community told the Nigerian Tribune that until she was contracted to work in Rann on a short-term basis Hauwa was a student of the health education department of the University of Maiduguri. At this hard time, the European Union stands by the Nigerian authorities and people.
It added that numerous IDPs were injured in the attack. Eight of the 11 are Nigerian security officials including four soldiers, worldwide agencies have said.
"We are outraged and saddened".
Meanwhile, President Muhammadu Buhari said the attack on humanitarian workers in Borno has shown the outlawed group as godless, brutish, and utterly to be despised.
But there was no immediate official confirmation of the casualty figures.
Nigeria's immediate past President Goodluck Jonathan and ex-Kaduna State governor, Senator Ahmed Makarfi have condemned the spates of killings in the country, particularly the recent attack that led to the death of global aid workers in Rann, Borno State.
Commanders at the time called the bombing a mistake and blamed "the fog of war".
The camp for displaced people in Rann was bombed in a Nigerian Air Force strike a year ago, killing up to 170 people.