The UN's refugee agency has called for better access to northern Syria's Raqqa province, saying close to half a million people are in need of assistance.
Kurdish-led forces attacked the provincial capital, also called Raqqa, a week ago, hoping to drive the militants out with the aid of US-led coalition airstrikes.
Clearing operations around the city have been under way for months, and the UNHCR says 100,000 people were displaced in May alone.
UNHCR spokesman Andrej Mahecic said the barriers to movement have made aid operations "costly and complex".
He said all land routes to the region have been blocked by other parties to Syria's civil war that are hostile to the US-backed force, forcing the aid agency to rely on airlifts.
"Resources are also badly needed," said Mr Mahecic. "Funding is not keeping up with needs on the ground."
The UN has managed to raise only $29m of the $153m it budgeted to meet humanitarian needs in Raqqa province.
Turkey, which views the main Kurdish militia taking part in the fight against IS as a terrorist group because of its links to Kurdish rebels, has sealed much of its border with northern Syria, disrupting aid operations and the movement of refugees. The Islamic State group has blocked humanitarian access to Raqqa from the south.
Human Rights Watch has, meanwhile, called on the US-led coalition to make protection of civilians a priority in the campaign to recapture Raqqa.
"We have already documented a series of rights abuses in the context of anti-ISIS operations," said Lama Fakih, the deputy Middle East director for Human Rights Watch, using another acronym for IS.
The New York-based group said in a statement that the United States and allied ground forces must respect the human rights of everyone caught up in the battle.
It also urged the US to investigate air strikes that have allegedly targeted civilians, respect detainee rights, provide safe passage for the displaced and intensify efforts to clear land mines. And it sought guarantees against enlisting child soldiers into the ranks of US partner forces.
The US is providing ground and air support to the Kurdish-led SDF in the battle for Raqqa, which has since 2014 been the Islamic State group's main base in Syria.
HRW reported in 2014 the SDF's leading faction, the People's Protection Units (YPG), had enlisted soldiers under the age of 18.