"If these channels, these lifelines, are not kept open it is catastrophic for people who are already in... the world's worst humanitarian crisis," Jens Laerke, a spokesman for the United Nations humanitarian office (OCHA), told reporters in Geneva, AFP reported.
Relief supplies into Yemen have been blocked since the coalition sealed off the country's borders following a missile attack last weekend by Yemen's Houthi.
Humanitarian flights into Yemen have been on hold since Monday.
The coalition has also asked the UN to clear ships from the port of Hodeidah, which "is the backbone of the humanitarian operations in Yemen," OCHA said in a statement.
"Fuel, food and medicine imports must continue to enter the country," Laerke said. "This is an access problem of colossal dimensions".
In the immediate aftermath of the blockade, fuel prices have jumped by up to 60 per cent and cooking gas prices have doubled, he added.
OCHA is in talks with the coalition to restore access as soon as possible, Laerke said.