A key member of Israel's war cabinet quit Prime Minister Benjamin Netanyahu's government on Sunday, heaping domestic pressure on the Israeli leader as the war in Gaza rages.

Benny Gantz, a former Israeli general and defence minister, announced his resignation from the emergency body after failing to get a post-war plan for Gaza approved by Netanyahu, which he demanded in May.

His departure is not expected to bring down the government, a coalition including religious and ultra-nationalist parties, but it marks the first political blow to Netanyahu eight months into the Gaza war against Palestinian Hamas militants.

"Netanyahu is preventing us from progressing to a real victory. That is why we are leaving the emergency government today with a heavy heart," Gantz said.

Gantz, who turned 65 on Sunday, has been seen as a favourite to form a coalition in the event that Netanyahu's government is brought down and early elections are called.

His centrist National Union Party submitted a bill last week to dissolve the Knesset, Israel's parliament, and hold early elections.

The former army chief, one of Netanyahu's main rivals before joining the war cabinet, had repeatedly called on Israel to reach a deal to secure the release of all hostages and to make it a "priority".

Since a week-long ceasefire in November, which saw the release of scores of hostages, Israel has failed to reach any further agreement and has kept on with its military campaign in Gaza.

"Israel has not made it a priority, clearly, so that was kind of the first major break when Gantz indicated that he would leave," said political analyst Mairav Zonszein.