At least 1,600 people dead in Hamas-Israel war as fighting enters fourth day.

Israel mobilized 300,000 reservists and continued airstrikes in Gaza City Tuesday morning as Palestinian families were seen carrying belongings down damaged streets with smoke rising from buildings. The Israeli military claimed it had regained control of the Gaza border.

President Biden announced at least 11 U.S. citizens were killed in the Hamas attacks over the weekend, and the President is expected to make an address Tuesday.

"While we are still working to confirm, we believe it is likely that American citizens may be among those being held by Hamas," Biden said Monday night, "I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts."

"We're in constant touch with the Israeli officials right now to try to get a sense of the whereabouts of all the unaccounted for Americans because who knows where they are or what condition they're in," said John Kirby, the National Security Council spokesman.

Israeli Prime Minister Benjamin Netanyahu promised a siege in retaliation for the Hamas attacks on Israeli civilians, saying "what we will do to our enemies in the coming days will reverberate with them for generations."

Hamas claimed it had taken civilian hostages and threatened to begin executions if Israel attacked civilian targets without warning.

The United Nations reported more than 187,500 people have been displaced in Gaza since the war broke out Saturday.

The U.N. Office for the Coordination of Humanitarian Affairs UNRWA says the U.N. is hosting more than 137,000 people in schools across the territory. The report says airstrikes have razed 790 housing units and severely damaged 5,330 in the territory of 2.3 million people. Also a concern, is damage to water, sanitation and hygiene facilities in Gaza that has disrupted service for more than 400,000 people.

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"We didn’t want this war It was forced upon us in the most brutal and savage way," said Israeli Prime Minister Benjamin Netanyahu on Monday.

The U.S. announced Monday that the USS Gerald R. Ford Strike Group is en route to the Mediterranean as a warning to Iran, Lebanese Hezbollah and others in the region that any intervention to escalate the attack on Israel will not be tolerated.

Kirby said, however, that "there is no intention to put U.S. boots on the ground."

Turkish President Recep Tayyip Erdogan held telephone calls with Palestinian President Mahmoud Abbas and Israeli President Isaac Herzog, according to Erdogan's press office.

Erdogan and Abbas discussed the ongoing conflict between Israel and the militant group Hamas. "President Erdogan stated that Turkey is making every effort to end the conflicts in the region and ensure calm as soon as possible" a statement from his press office said.

In a joint statement Monday, President Biden and four other leaders of France, Germany, Italy, and the United Kingdom said, "All of us recognize the legitimate aspirations of the Palestinian people, and support equal measures of justice and freedom for Israelis and Palestinians alike. But make no mistake: Hamas does not represent those aspirations, and it offers nothing for the Palestinian people other than more terror and bloodshed."

Arab foreign ministers plan to convene Wednesday in Cairo for a meeting on the war between Israel and the Palestinian militant groups in Gaza.

Arab League Assistant Secretary-General Hossam Zaki said the ministers would discuss Arab efforts to "stop the Israeli aggression" on Gaza. The meeting was called by the Palestinians.