U.N. spokesman Stephane Dujarric told reporters the executions and abductions happened Monday in northern Borno state on the Damaturu-Biu road linking Yobe and Borno states. He gave no other details.
The United Nations and its humanitarian partners condemned the violent incidents and urged Nigerian authorities to do their utmost to prevent further violence and protect civilians.
Borno state was the birthplace of the Boko Haram insurgency a decade ago and it has suffered the worst of the Boko Haram attacks.
Dujarric said over 36,000 people have been killed since the beginning of the conflict, about half of them civilians.
U.N. Secretary-General Antonio Guterres expressed deep concern at the reports of civilian executions and abductions and called for those responsible to be brought to justice, Dujarric said.
The U.N. chief sent condolences to the victims and reiterated U.N. solidarity with the people and government of Nigeria, he said.
"The secretary-general recalls that attacks by a party to an armed conflict that target civilians, aid workers and civilian infrastructure violate international humanitarian law," Dujarric said.
"Those responsible for these atrocities must be held accountable," he said. "International human rights law and international humanitarian law must be fully respected, and all civilians in Nigeria must be protected."
Antonio José Canhandula, the acting U.N. humanitarian chief in Nigeria, said in a statement Monday that reports indicated there were attacks on the Monguno-Maiduguri Road in northern Borno state, as well as on the road link with Yobe state.
He said he was "horrified" by the reports, and was still gathering information.
Canhandula said aid workers in the region condemned the incidents and what he described as "the increasing practice" by armed groups of setting up checkpoints targeting civilians.
"It is urgent for the Nigerian authorities to do their utmost to prevent further violence and brutality and to protect the civilian population, including aid workers, from such grave violations of international laws, especially women and children who are among the most vulnerable and are caught up in the violence," he said.
Canhandula reported that there has been an upsurge in violence, particularly along main roads over the past six months, which has resulted in a deterioration in the humanitarian situation.
This year, he said, more than 160,000 people fled their homes for shelter in already congested camps.