Four children on the beach in Gaza City were killed in an Israeli raid on Wednesday, raising the overall death toll to 213, the emergency services said.
All four were on the beach when the attack took place, in what initially appeared to be shelling from the sea, according to AFP correspondents who witnessed the attack from a nearby hotel.
Several injured children took refuge at the hotel, they added.
The latest violence took Wednesday's death toll to 16, and raised the overall number of dead in Gaza after nine days of violence to 216, emergency services spokesman Ashraf al-Qudra said. More than 1,560 have been wounded.
Earlier, a 37-year-old man was killed in an air strike on the Zeitun neighbourhood, east of Gaza City, which took place several hours after the army warned 100,000 residents to leave the area and the neighbouring district of Shejaiya.
Most of Wednesday's deaths came in the south, with six killed in several strikes on Khan Yunis and five killed in Rafah, which straddles the Egyptian border.
In Khan Yunis, four people from the Al-Daqqa family were killed in two separate strikes, one of which killed a 65-year-old woman and a 10-year-old boy.
One of the victims in Khan Yunis was killed by tank fire.
In Rafah, five men were killed in four separate strikes, one of which hit a house in the city, Qudra said.
Earlier, Hamas officially told Egypt that it rejects an Egyptian-proposed Gaza ceasefire, a spokesman for the Palestinian group said on Wednesday.
"The outcome of discussions within the internal institutions of the movement was to reject the proposal and therefore, Hamas informed Egypt last night it apologises for not accepting it," spokesman Sami Abu Zuhri said.
Israel's security cabinet accepted the ceasefire proposal on Tuesday, a government spokesman said, after a week of the deadliest violence in and around Gaza since fighting broke out in 2012. However, the Israeli Defense Forces (IDF) resumed attacks Tuesday afternoon after the truce failed to get off the ground, AFP correspondents and eyewitnesses said.
"Following six hours of indiscriminate rocket fire at Israel, the IDF have resumed operational activities against Hamas," military spokesman Lieutenant Peter Lerner said on his Twitter account.
According to figures provided by the Gaza-based Palestinian Centre for Human Rights (PCHR), more than 80 percent of the victims were civilians.
So far, one person has been killed in Israel -- a civilian who died on Tuesday evening in a rocket strike near the Erez crossing, medics said. Four Israelis have been seriously wounded.
During its operation, Israel has struck more than 1,750 "terror targets" across the coastal enclave, the army said.