At least 37 Palestinians, including women and children, were killed early on Saturday morning by Israeli attacks east of Gaza's Khan Younis and in the Al-Nuseirat camp area, Al Jazeera is reporting.
Many others were injured in the attacks.
Meanwhile, the United States said on Friday said that progress had been made at the latest round of Gaza truce talks, after the presence of Israeli troops on the Egyptian border emerged as a major sticking point.
The White House said CIA chief William Burns was among US officials taking part in the discussions in Cairo, joining the heads of Israel's spy agency and security service.
"There has been progress made. We need now for both sides to come together and work towards implementation," US National Security Council spokesman John Kirby said.
Preliminary talks that began Thursday "were constructive in nature", he said, adding that reports that the diplomacy was "near collapse" were inaccurate.