CAIRO/GAZA/JERUSALEM, Dec. 13 (Reuters) - Israel announced its worst combat losses in more than a month on Wednesday after an ambush in the ruins of Gaza, and faced growing diplomatic isolation as civilian deaths mounted and a humanitarian catastrophe worsened in the Palestinian territory.

Intense fighting between Hamas militants and Israeli soldiers was under way in both north and south Gaza, a day after the United Nations demanded an immediate humanitarian ceasefire and U.S. President Joe Biden said Israel's "indiscriminate" bombing of civilians was costing international support.

Israeli Prime Minister Benjamin Netanyahu said the military would fight on despite international pressure for a ceasefire.

"We're continuing until the end, until victory, until Hamas is annihilated," he told soldiers in Gaza over radio. "I say this in the face of great pain but also in the face of international pressures. Nothing will stop us."

Israel reported 10 of its soldiers killed in the past 24 hours, including a full colonel commanding a forward base and a lieutenant-colonel commanding a regiment. It was the worst one-day loss since 15 soldiers were killed on Oct. 31.

Most of the deaths came in the Shejaia district of Gaza City in the north, where troops were ambushed trying to rescue another group of soldiers who had attacked Hamas fighters in a building, the military said.

Hamas said the episode showed that Israeli forces could never subdue Gaza: "The longer you stay there, the greater the bill of your deaths and losses will be, and you will emerge from it carrying the tail of disappointment and loss, God willing."

In a televised address, Hamas Chief Ismail Haniyeh said any future arrangement in Gaza without Hamas was a "delusion".