By Gabrielle Tétrault-Farber and Emma Farge

GENEVA The blockaded Gaza Strip still has some fuel to operate generators such as those in hospitals but it could run out in hours, a Red Cross official said on Thursday, saying it is too unsafe to move stored aid around the enclave.

Israel has responded to Saturday's deadly surprise attack by Hamas militants by putting Gaza, home to 2.3 million people, under total siege and launching by far the most powerful bombing campaign in the 75-year history of the Israeli-Palestinian conflict.

"Our understanding is that there is still fuel - but probably only for a few hours - to allow generators to work, including in hospitals," Fabrizio Carboni, the International Committee of the Red Cross (IRCRC) regional director, told reporters.

Carboni said in a later interview that its stocks of aid and medicine within Gaza are stranded for want of safe passage. "We still have supply in Gaza. The problem is that we can't move."

Talks are ongoing with all parties, including Egypt, to open a humanitarian corridor, with the Rafah crossing between Gaza and Egypt's Sinai peninsula the main option being considered, he said.

"With the supplies we have inside Gaza, we will not be able to support the civilian population for very long. This is why we need to be able to get the supplies we need to Gaza," he said.

Most of Gaza's population has no power and water as Israeli strikes continue and they fear a possible ground invasion.

Asked whether the ICRC was preparing for this scenario, Carboni said: "We are preparing, yes, for the worst. Now the worst in the region has no limits so it's difficult".

ICRC teams are pre-positioning staff and stocks in strategic hubs close to Gaza as part of its preparations, he added.

Israel has said there will be no humanitarian exceptions to its siege until all its hostages seized by Hamas militants during the weekend attacks were freed. The attacks were the deadliest on Jewish civilians since the Holocaust and public broadcaster Kan said the toll had risen to more than 1,300.

The ICRC, an impartial humanitarian body which visits prisoners of war and detainees and sometimes facilitates their release, said earlier it was in direct contact with both Hamas and Israeli officials about the hostages but declined to give details.

(Reporting by Gabrielle Tétrault-Farber, Emma Farge and Maya Gebeily; Editing by Alex Richardson, Nick Macfie, Alexandra Hudson)