International medical NGOs issued a stark warning Tuesday over the health and humanitarian situation in the Gaza Strip, which has been bombed by the Israeli military following Hamas's weekend attack.

The groups called for a humanitarian corridor to support medical relief efforts, and respect for humanitarian law.

"The situation is catastrophic… I don't think anyone is safe in Gaza," said Sarah Chateau, head of the Palestinian territories programme for Doctors Without Borders (MSF).

MSF has been working in the region for more than 20 years, employing 300 Palestinians and 20 international staff.

"We transferred our teams to a United Nations building. The bombardments were so massive that the risks were too great", she added.

Since Saturday's unprecedented Hamas attack, which left hundreds dead, the Israeli army has been bombarding the Gaza Strip, controlled by the Palestinian militant group Hamas since 2007.

In four days, the war has already claimed thousands of lives.

Israel has imposed a "total siege" on the Gaza Strip and ordered the "immediate cut-off" of water supplies to the Palestinian enclave, following the suspension of electricity and food deliveries.

Hamas is threatening to execute hostages kidnapped in Israel, believed to be around 150 people, including children, women and young people captured while at a music festival.

"With a total state of siege, how long will our teams be able to hold out? We need a humanitarian corridor to support the medical response, to bring in equipment, to replace the teams on the ground," Chateau said.

"Transporting patients is reduced by the blockade and the intensity of the bombardments. Our team is risking their lives. and it's very difficult for them to carry out their work", said Jean-Francois Corty, vice-president of Medecins du Monde, another NGO present in Gaza.

He warned that "80 percent of the population is dependent on humanitarian aid".