At least 150 Israelis and foreigners have been held captives in the Gaza Strip since Hamas's surprise Saturday attack on Israel.

"As a neutral intermediary, we stand ready to conduct humanitarian visits, facilitate communication between hostages and family members, and to facilitate any eventual release," Fabrizio Carboni, the ICRC's regional director for the Near and Middle East, said in a statement.

Hostage-taking is forbidden under international humanitarian law, and anyone detained must be released immediately, Carboni added.

The ICRC urged "both sides to reduce the suffering of civilians".

In retaliation for Saturday's attack, Israel has relentlessly pounded Gaza and imposed a complete siege on the impoverished territory of over 2 million people, cutting off water, electricity and fuel supplies.

Hamas has claimed that four captives died in Israeli strikes and threatened to kill others if civilian targets are bombed without advance warning.

Turkish President Recep Tayyip Erdogan has also started negotiations with Hamas to release the captives, an official source told AFP late Wednesday.

The war has already claimed several thousand lives on both sides since Saturday.

"The human misery caused by this escalation is abhorrent," Carboni said.

As Gaza is deprived of electricity, "hospitals lose power, putting newborns in incubators and elderly patients on oxygen at risk. Kidney dialysis stops, and X-rays can't be taken", he added.

"Without electricity, hospitals risk turning into morgues."

Drinking water, already difficult to access, has also become more scarce.

"No parent wants to be forced to give a thirsty child dirty water," he said.

More than 338,000 people have been forced to flee their homes in Gaza, according to the UN. At the same time, the European Union has called for a "humanitarian corridor" to allow civilians to flee Gaza's fifth war in 15 years.