Valerie Amos, undersecretary-general for Humanitarian Affairs and Emergency Relief Coordinator, urged all sides involved to refrain from violence and to guard civilians' safety.
"I am especially concerned about the humanitarian situation in Yemen because, even before the recent protests, the country was facing a humanitarian crisis due to protracted conflict in the north displacing 300,000 people, some of them multiple times," she said.
"The recent fighting has again affected hundreds of people that have not recovered from earlier conflict."
The U.N. Office for the Coordination of Humanitarian Affairs says Yemen is facing acute water and food shortages, as well.
U.N. officials estimate more than 80 people have died and hundreds have been injured during the country's recent unrest.
Yemeni President Ali Abdullah Saleh has told the General People's Congress, Yemen's ruling party, no more concessions will be made to resolve the popular uprising.
Saleh said Sunday concessions offered so far to rebels were made in coordination and agreement with the GPC's leadership, the state-run SABA news organization reported.
Saleh also called for dialogue and an agreement on a peaceful transfer of power. He also warned of sectarian strife if the opposition continues to reject solutions he has offered, SABA said.
"If we stand up together to face this challenge, the crisis will end," Saleh said.
Last week, Saleh told partisans he was willing to surrender power, but only to "safe hands." His statement, which indicated no time frame for a power transfer, came as he was involved in negotiations on the timing and conditions for ending his 32-year rule.
U.N. Secretary-General Ban Ki-moon also has voiced concerns about the situation in Yemen, calling for a broad dialogue involving the political opposition, youth groups and other elements of the country's society to reach agreement on "bold" reforms.