The United Nations deplored the resumption of conflict in Gaza on Friday, describing the hostilities as "catastrophic" and urging parties to bring about a lasting ceasefire.

Jens Laerke, spokesperson for the United Nations humanitarian office in Geneva, said the resumption of hostilities meant "hell on Earth has returned to Gaza".

Israeli warplanes resumed bombing Gaza, sending Palestinian civilians fleeing for shelter, after a week-old truce ran out with no deal to extend it.

"The resumption of hostilities in Gaza is catastrophic," said Volker Turk, the United Nations High Commissioner for Human Rights.

"I urge all parties and states with influence over them to redouble efforts, immediately, to ensure a ceasefire – on humanitarian and human rights grounds."

In a post on X social media platform, United Nations Secretary-General Antonio Guterres said he regretted the resumption of hostilities and hoped a new pause could be established.

"The return to hostilities only shows how important it is to have a true humanitarian ceasefire," he said.

Rockets are launched from the Gaza Strip into Israel, after a temporary truce expired between Israel and the Palestinian Islamist group Hamas, as seen from Israel's border with Gaza in southern Israel.

Laerke said that the week-long truce had seen significantly larger humanitarian convoys entering densely populated Gaza, even reaching north of Wadi Gaza, which prior to the pause had received almost no supplies.

"With the resumption of war, we fear that the continuation of this (aid) is now in doubt," he said.

"The Rafah crossing is closed as of now. We need a resumption of a humanitarian pause, not a return to war."

Appealing for a lasting ceasefire, UNICEF called inaction on Gaza "an approval of the killing of children".

"A lasting ceasefire must be implemented," James Elder, spokesperon for UNICEF, told reporters via video link from Gaza.

"Inaction at its core is an approval of the killing of children.... It is reckless to think more attacks on the people of Gaza will lead to anything other than carnage."