The head of medical charity Médecins Sans Frontières (MSF) said on Thursday that Gaza faces a catastrophe extending far beyond a humanitarian crisis, describing the situation in the densely populated enclave as chaotic.

Israeli forces battled Hamas militants in the Gaza Strip's biggest cities on Thursday in a new phase of the war that is now entering its third month, with wide areas of the narrow territory flattened by Israeli bombardment and 85% of the 2.3 million population left homeless, according to U.N. figures.

"My people on the ground keep updating me on the situation, and I can tell you that it has gone far beyond the humanitarian crisis," Dr Christos Christou, international president of Doctors Without Borders, told reporters in Geneva.

"It is a humanitarian catastrophe. It is a chaotic situation, and I'm extremely worried that very soon people will be in a mode of just trying to survive, which will come with very severe consequences."

In a bid to escape Israeli bombardment, Gazans have amassed at the southern tip of Gaza, heeding Israeli leaflets and messages saying that they would be safe on the border with Egypt. The United Nations and aid organisations have said that nowhere is safe in Gaza.

"The people have been asked to be squeezed in a very small area," Christou said. "My teams on the ground keep saying to me that it is unbearable. It is unsustainable ... There is no safe place."

In an open letter to the U.N. Security Council published on Monday, Christou implored the body to demand an end to Israeli attacks against Palestinian civilians and allow humanitarian aid to enter Gaza unimpeded.

Israel says it does its utmost to minimise civilian casualties but that Hamas combatants use built-up residential areas for cover, something the Islamist militant group denies.

(Reporting by Gabrielle Tétrault-Farber; Editing by Mark Heinrich)