An Israeli air strike in southern Gaza hours before a humanitarian truce was declared killed 20 people, including 11 children, most of them from a single family, medics said.

The strike in southern Khan Yunis hit the home of the Najjar family, killing at least 14 members of the clan, emergency services spokesman Ashraf al-Qudra said.

Another two members of the Abu Shahla family were killed, along with four other people, two of whom have yet to be identified.

The dead included 11 children, among them a one-year-old girl and a three-year-old boy, Qudra said.

The strike raised the toll in Gaza on the 19th day of violence between Israel and Hamas to at least 891 people, before a fragile 12-hour ceasefire went into effect.

The toll is expected to rise as bodies are pulled from the rubble of homes in some of the worst-hit parts of Gaza, in northern Beit Hanun, eastern Shejaiya and Zaitun, and southern Khan Yunis.

The conflict that began on July 8 when Israel launched an operation to stamp out rocket fire from Gaza and destroy Hamas tunnels has also killed 37 Israeli soldiers, and three civilians inside Israel.