A Palestinian man carries an injured boy following the resumption of Israeli bombardment in Rafah, southern Gaza Strip, on December 1, 2023.AFP
Israel resumed its bombardment in Gaza on Friday immediately after the expiry of a week-long truce, with the first fatalities reported minutes later, according to health officials in the Palestinian territory.
"I deeply regret that military operations have started again in Gaza," Guterres said on X, formerly Twitter.
"I still hope that it will be possible to renew the pause that was established. The return to hostilities only shows how important it is to have a true humanitarian ceasefire."
The United Nations human rights chief said the resumption of hostilities 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.
Meanwhile, the United Nations children's agency UNICEF condemned the resumption of fighting, saying: "Today, those in power have decided that the killing of children would recommence."
"The humanitarian situation in Gaza is so perilous that anything other than sustained peace and at-scale emergency aid will mean catastrophe for the children of Gaza," UNICEF spokesman James Elder told reporters in Geneva via video link from the besieged Palestinian enclave.
Elder said the "dire" state of health, nutrition, water, and sanitation there threatened to trigger "a disaster of unparalleled magnitude for the children of Gaza".
"Inaction, at its core, is an approval of the killing of children," he said, calling it "reckless to think more attacks on the people of Gaza will lead to anything other than carnage."
"It's deeply unsettling to hear how some have been able to overlook the tragic deaths of these children; and now, today, are seemingly comfortable with the horrors -- the attacks -- starting again," he said.
"To accept the sacrifice of the children in Gaza is humanity giving up.
"This is our last chance before we delve into yet again explaining why another entirely avoidable tragedy was not stopped."
Elder said that he visited children in the Nasser Hospital in Khan Younis in the southern Gaza Strip as hostilities resumed on Friday.
He said that during the truce, "slowly, slowly, glimpses of childhood started to come back" to traumatized and wounded children.
However, as the explosions began, "in an instant... you saw fear return and childhood retreat", he said.