Missiles have struck several sites in Gaza, including a park inside a refugee camp and an outpatient building of the strip's largest hospital, disrupting a relative lull at the start of the Muslim Eid al-Fitr holiday.
Eight people, including seven children, died following missile fire on a park inside the Shati refugee camp on the edge of Gaza City, medics said.
The children were playing on a swing when the strike hit the park, Ayman Sahabani, the head of the emergency room at Shifa hospital, told reporters.
Munzer al-Derby, 35, who witnessed the strike, told Al Jazeera: "The kids were playing on the wheel... A rocket fell and cut them apart."
"I know some of them. They were from Al-Helou family who left their homes in Shujayea (east Gaza city, where massive artillery fire destroyed neighbourhoods). They came here and rented an apartment last week," al-Derby said.
The Israeli army swiftly denied it was behind the strike, tweeting that a misfired rocket from Gaza had hit the playground.
"We had no activity in the area. We know it was launched from within Gaza and landed short," Lieutenant Colonel Peter Lerner, an Israeli army spokesman, said.
However, Hamas denied it had fired any rockets in the area and said it was "categorically an airstrike by Israel". It said it had collected schrapnel from the scene that it could prove was from an Israeli munition.
Medics said that an Israeli missile also hit a building, believed to be an outpatient clinic, close to the main gate of Shifa hospital, the same hospital where the victims of the playground strike were taken.
Al Jazeera's Imtiaz Tyab, reporting from the hospital, said there were chaotic scenes as "a number of small bodies were brought into this hospital".
"It's believed that because it's been relatively calm, many of these children went outside to enjoy themselves on this Eid holiday but tragically they've been killed," Tyab said.
At least another five Palestinians, including three children, were killed in other attacks on Monday. A four-year-old boy died when tank shells hit his family's house in Jabalia, in the northern Gaza Strip, Gaza health officials said. Another person was killed by tank shelling in a separate incident, also in Jabalia.
Monday’s violence followed an almost 12-hour pause in fighting and came as international efforts intensified to end the three-week war between Israel and Hamas.
The United Nations on Monday called for an "immediate" ceasefire in the fighting that has already killed more than 1,040 Palestinians, 43 Israeli soldiers and three civilians on the Israeli side.
But Benjamin Netanyahu, the Israeli prime minister, warned Israelis later that evening to "be prepared for a lengthy campaign" after four soldiers were killed in a mortar attack at Eshkol in southern Israel.
"Israeli citizens cannot live with the threat from rockets and from death tunnels - death from above and from below," Netanyahu said in a speech broadcast live, soon after the news of the shelling of the Eshkol region that also reportedly wounded at least 12 people.
The military said at least a dozen rockets had been fired from Gaza at Israel since midnight
Eid of mourning
As Muslims began celebrating Eid al-Fitr, there was fear and mourning on Monday instead of holiday cheer in large parts of Gaza.
Palestinian families huddled inside their homes, fearing more airstrikes, while those who came to a cemetery in Gaza City's Sheik Radwan neighbourhood to pay traditional respects at their ancestors' graves gathered around a large crater from an airstrike a week ago that had broken up several graves.
Amid an eerie calm, the call to Eid prayer echoed in the southern town of Rafah on Monday morning. Dozens of worshippers lined the rows of a severely destroyed mosque, with a collapsed roof and missing walls. Many of the faithful looked sombre during the traditional holiday sermon.
In Gaza City, dozens of men prayed in the courtyard of a UN school surrounded by school desks. Children and women stood on a higher level overlooking the worshippers.
"We are suffering and will suffer but we need our rights, our houses, our lands and our farms to return to us and we will not accept living a miserable life," said Abu Saber Jalees, who fled fighting to seek shelter at the school.
Meanwhile, in New York, an emergency meeting of the UN Security Council called for "an immediate and unconditional humanitarian ceasefire".
And while it was the council's strongest statement yet on the Gaza war, it was not a resolution and therefore not binding.