Israeli troops could be seen clearing ground and building watch towers on Monday in parts of Gaza they have seized in recent days in a renewed offensive that the United Nations says has already captured or depopulated two-thirds of the enclave.

An Israeli airstrike on a residential block in northern Gaza killed at least 23 people, health officials in the Palestinian territory say.

The Israeli military says it had struck a "senior Hamas terrorist" in the Shejaiya neighbourhood of Gaza City.