Mourners react next to the bodies of Palestinians killed in Israeli strikes, at Al-Aqsa Martyrs hospital in Deir Al-Balah, in the central Gaza Strip. /Ramadan Abed/Reuters

Israeli airstrikes pounded Gaza, killing more than 400 people, Palestinian health authorities said on Tuesday, threatening the complete collapse of a two-month ceasefire as Israel vowed to use more force to free hostages held by Hamas.

Shortly after 1000 GMT, officials from Gaza's Hamas-run health ministry put the death toll at 404, but it is highly likely to rise. 

Hamas, which still holds 59 of the approximately 250 hostages seized in its October 7, 2023 attack on Israel, accused Israel of breaching the ceasefire and jeopardizing efforts by mediators to secure a permanent truce.

Israel's prime minister, Benjamin Netanyahu, said he had told the military to take "strong action" against Hamas in response to the group's refusal to release the remaining hostages and because of their rejection of ceasefire proposals.

Egypt, one of the mediators in the ceasefire deal agreed in January, called for restraint and urged all parties to work towards a lasting agreement.

'A night of hell'

Airstrikes hit houses and tent encampments from the north to south of the Gaza Strip and Israeli tanks shelled across the border line into the east and south of the enclave.

"It was a night of hell. It felt like the first days of the war," said Rabiha Jamal, 65, a mother of five from Gaza City.

"We were preparing to have something to eat before starting a new day of fasting when the building shook and explosions began. We thought it was over but war is back," she said.

In hospitals strained by 15 months of bombardment, piles of bodies in white plastic sheets smeared with blood could be seen stacked up as casualties were brought in. Officials from Gaza's Hamas-run health ministry said many of the dead were children.

The Israeli military said it hit dozens of targets, and that the attacks would continue for as long as necessary and extend beyond airstrikes, raising the prospect that Israeli ground troops could resume fighting.

'Tragedy onto tragedy' - UN

The UN human rights chief Volker Turk said he was horrified by the Israeli bombardment.

"This will add tragedy onto tragedy," he said in a statement. "Israel's resort to yet more military force will only heap further misery upon a Palestinian population already suffering catastrophic conditions."

Israel has halted aid deliveries into Gaza for over two weeks, exacerbating a humanitarian crisis.

Israeli media said Israel was opening shelters in multiple areas in commercial hub Tel Aviv to prepare for possible retaliation from Hamas or Yemen.

Israel's renewed intense pressure on Hamas came as tensions flared elsewhere in the Middle East, which has seen the Gaza war spread to Lebanon, Yemen and Iraq.

The attacks were far wider in scale than the regular drone strikes Israel has said it has conducted recently against suspected militants, and follow weeks of failed efforts to agree an extension to the truce agreed on January 19.

Witnesses in Gaza said Israeli tanks shelled areas in Rafah in the southern Gaza Strip, forcing many families who had returned after the ceasefire to leave their homes again and head north to Khan Younis.

Among those killed was Mohammad Al-Jmasi, a senior member of Hamas' political office, and members of his family, including his grandchildren who were in his house in Gaza City when it was hit by an air strike, Hamas sources and relatives said. In all, at least five senior Hamas officials were killed along with members of their families.

Kremlin concern over civilian casualties

The Kremlin said on Tuesday it was concerned by the reports. "Undoubtedly, it's another deterioration in the situation (in Gaza) and another spiral of escalation that is causing our concern," Kremlin spokesman Dmitry Peskov told reporters.

"Especially concerning of course are the reports of major casualties among the civilian population," Peskov added. "We are monitoring the situation very closely and, of course, we are waiting for it to return to a peaceful course."

China's Ministry of Foreign Affairs spokeswoman Mao Ning expressed a wish to avoid a "humanitarian disaster."

"China is closely watching the current Palestine-Israel situation," she told a news conference. "We hope that all parties can push for the continuous and effective implementation of the ceasefire agreement, and avoid any actions that may escalate the situation, so as to prevent a larger-scale humanitarian disaster."

In Washington, the White House said Israel had consulted the U.S. administration before it carried out the strikes.

"Hamas could have released hostages to extend the ceasefire but instead chose refusal and war," White House spokesperson Brian Hughes said.

Ceasefire negotiations

Negotiating teams from Israel and Hamas had been in Doha as mediators from Egypt and Qatar sought to bridge the gap between the two sides after the end of an initial phase in the ceasefire, which saw 33 Israeli hostages and five Thais released in exchange for some 2,000 Palestinian prisoners.

With the backing of the United States, Israel had been pressing for the return of the remaining hostages in exchange for a longer-term truce to halt fighting until after the Muslim fasting month of Ramadan and the Jewish Passover holiday in April.

However, Hamas has insisted on moving to negotiations for a permanent end to the war and a full withdrawal of Israeli forces from Gaza, in accordance with the terms of the original ceasefire agreement.

On Tuesday, Hamas spokesperson Abdel-Latif Al-Qanoua said communication with mediators was ongoing, and the group was keen to complete the implementation of the original Gaza ceasefire deal.

Much of Gaza now lies in ruins after the war, which erupted on October 7, 2023 when Hamas-led gunmen attacked Israeli communities, killing some 1,200 people, according to Israeli tallies, and abducting 251 hostages into Gaza.

The Israeli campaign in response has killed more than 48,000 people in Gaza, according to Palestinian health authorities, and destroyed much of the housing and infrastructure in the enclave, including the hospital system.