Israel's military says it has warned medical officials and international organisations to prepare for the planned evacuation of Gaza City's one million residents ahead of an offensive to occupy it.

The officials were told that "adjustments" were being made to hospitals in southern Gaza to receive patients, a statement said.

Gaza's Hamas-run health ministry rejected "any step that would undermine what remains of the health system".

The UN and aid groups have also vowed to stay to help those who cannot or choose not to move.

Meanwhile, Palestinians said there had been heavy bombardment in eastern areas of the city, a day after the military said it had taken the first steps in the offensive.

Israeli Prime Minister Benjamin Netanyahu met security officials on Thursday to approve the military's takeover plans despite widespread international and domestic opposition.

He announced Israel's intention to conquer the entire Gaza Strip after indirect talks with Hamas on a ceasefire and hostage release deal broke down last month.

The Israeli military plans to evacuate Gaza City's entire population and move it to shelters in the south before troops move into the territory's largest urban area.

As part of its preparations, it said, officers from military body Cogat carried out "initial warning calls" to medical officials and international organisations on Tuesday.

"The officers emphasised to the medical officials that adjustments are being made to the hospital infrastructure in the south of the Strip to receive the sick and wounded, alongside an increased entry of necessary medical equipment," a statement said.

It quoted the officers as telling them in the calls: "We are going to provide you with a place to be in, whether it is a field hospital or any other hospital."

However, Gaza's health ministry expressed its "rejection of any step that would undermine what remains of the health system after the systematic destruction carried out by the occupation [Israeli] authorities".

"Such a step would deprive more than one million people of their right to medical treatment and expose the lives of residents, patients, and the wounded to imminent danger," it warned.

Eighteen of Gaza's 36 hospitals are currently partially functional, according to the UN. Eleven of them are located in Gaza City governorate and one is in North Gaza governorate.