UNITED NATIONS - Flour and vegetable prices have increased more than 100-fold in the Gaza Strip and essential living materials await delivery following Israel's closure of the crossings into the enclave, UN humanitarians said on Monday.

"The Kerem Shalom, Erez and Zikim crossings have been closed for cargo," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said. "This means that vital humanitarian assistance, including thousands of tents, remains undelivered."

OCHA said its partners were assessing available stocks within the Gaza Strip.

Israel has blocked humanitarian aid to Gaza since early Sunday. Israeli Prime Minister Benjamin Netanyahu said the measure was taken to pressure Hamas in accepting a new proposal to extend the first phase of the ceasefire and hostage release deal, which he said was put forth by the United States.

Tom Fletcher, the UN emergency relief coordinator, said on social media Sunday that Israel's decision to halt aid into Gaza is "indeed alarming."

"International humanitarian law is clear: we must be allowed access to deliver vital lifesaving aid," he said. "We need to get aid in and the hostages out."

UN Secretary-General Antonio Guterres urged all parties to make every effort to prevent a return to the hostilities in Gaza, and called for humanitarian aid to flow back into Gaza immediately and for the release of all hostages, his spokesman said at the daily briefing.

The UN Children's Fund also warned that the stoppage of aid deliveries into Gaza will quickly lead to devastating consequences for children and families who are struggling to survive.