Gaza's Hamas-run health ministry says an Israeli air strike killed at least 70 people in the Al-Maghazi refugee camp in the centre of the strip.
A spokesman said the death toll was likely to rise given the large number of families living in the area.
It comes as Israeli and Arab media say Egypt, which borders the Gaza Strip, has put forward a new proposal for a ceasefire between Israel and Hamas.
Dozens of injured people were rushed from Maghazi to nearby Al-Aqsa Hospital with footage showing some children's faces covered in blood and body bags piled outside.
The health ministry says three houses were hit in the attack late on Sunday.
According to ministry spokesman Ashraf al-Qudra, a densely populated residential block was destroyed.