The Libyan government has promised the UN access to the besieged rebel city of Misrata.
But even as the announcement was made, Gaddafi's troops continued to pound rebel enclaves with shells and rockets.
The Libyan authorities have not guaranteed a halt of hostilities during any mercy mission, according to the UN's humanitarian chief, Valerie Amos, who met with the Libyan prime minister in Tripoli.
Ms Amos said she would try to send a team to the city of 300,000 as quickly as possible.
Residents and hospital officials in the city have described heavy shelling over the weekend, and said 17 people were killed on Sunday.
UN Secretary-General Ban Ki-moon appealed to Libyan forces to hold their fire.
"Considering the magnitude of this crisis . . . it is absolutely necessary that Libyan authorities stop the fighting, stop killing people," he told a news conference in Budapest, Hungary.
The UN has already set up an aid operation in Benghazi, the rebel stronghold in the eastern part of Libya they control. As part of Sunday's agreement with the Gaddafi government, the UN would be allowed to set up a presence in Tripoli as well, Ms Amos said.