Israel's military on Sunday ordered more areas in and around Gaza's second-largest city of Khan Younis to evacuate, as it shifted its offensive to the southern half of the territory where it says many Hamas leaders are hiding.

Heavy bombardments were reported overnight and into Sunday in the area of Khan Younis and the southern city of Rafah, as well as parts of the north that had been the focus of Israel's blistering air and ground campaign.

Israeli air strikes killed two Syrian pro-Hezbollah fighters when they hit sites belonging to the Iran-backed group near Damascus early on Saturday, a war monitor told AFP.

The strikes near Damascus came less than 24 hours after the end of a Gaza truce between Hezbollah ally Hamas and Israel.

Israel carried out deadly bombardments in Gaza for a second day on Saturday, hitting hundreds of targets, after a week-long truce with Hamas militants collapsed despite international calls for an extension.

As world leaders gathered for the COP28 climate summit in Dubai, the collapse of a cease-fire in the Israel-Hamas war Friday plunged the conflict back into open combat and cast a long shadow over the talks.

Israel's top diplomat for the Middle East huddled with leaders at the summit as his colleagues went through a book of posters of those held hostage by the militant group Hamas, placing yellow "released" stickers by some while looking at others still held. Meanwhile, just a street away at the Palestinian territories' first-ever pavilion, an official gave a horrified look when Associated Press journalists gave her the news that Israeli airstrikes and ground fighting in the Gaza Strip had resumed.

The United Nations human rights chief said the resumption of hostilities between Israel and Hamas in the Gaza Strip on Friday was "catastrophic", with the situation now "beyond crisis point".

"The resumption of hostilities in Gaza is catastrophic. I urge all parties and states with influence over them to redouble efforts, immediately, to ensure a ceasefire -- on humanitarian and human rights grounds," Volker Turk said in a statement.

Iranian delegates walked out of U.N. climate talks in the United Arab Emirates on Friday in protest over the presence of Israeli representatives, state media reported.

The Iranian side considered Israel's presence at COP28 "as contrary to the goals and guidelines of the conference and, in protest, it left the conference venue", Energy Minister Ali Akbar Mehrabian, who headed the Iranian delegation, was quoted as saying by the official news agency IRNA.

Mediator Qatar said Friday that efforts are continuing to renew an Israel-Hamas truce and expressed “deep regret” over the resumption of Israeli bombardments after a weeklong cease-fire expired earlier in the day.

Israel and Hamas have traded blame, with each saying the other side violated the terms of the truce. Qatar, which has served as a mediator along with Egypt, appeared to be singling out Israel’s role in the resumption of violence.

Israeli fighter jets hit targets in the Gaza Strip minutes after a weeklong truce expired on Friday, as the war with Hamas resumed in full force. Black smoke billowed from the besieged territory and Israel dropped leaflets over parts of southern Gaza urging people to leave their homes, suggesting it was preparing to widen its offensive.

Renewed hostilities heightened concerns for the about 140 hostages who remain in Gaza, after more than 100 were freed as part of the truce.

A blast and gunfire killed at least 11 civilians in eastern Iraq, two security officials said on Friday, an attack which the provincial governor blamed on jihadists.

The violence, in Diyala province, occurred on Thursday evening and targeted a minibus returning the civilians from an electoral meeting organized by a candidate from their tribe, an interior ministry official, who asked for anonymity, told AFP.

The Hamas-run health ministry in the Gaza Strip said 109 people had been killed in the Palestinian territory on Friday after a truce between Israel and Hamas expired and fighting resumed.

Spokesman Ashraf al-Qudra said in a statement that the military had also tallied "hundreds of wounded in the Israeli aggression".