Six people were killed and dozens were injured when a Gaza school sheltering refugees was hit by Israeli airstrikes, the United Nations Relief and Works Agency said.

At least 4,000 people have taken refuge in the school, UNRWA said.

The Israel Defense Forces said Tuesday that it's "preparing to implement a wide range of offensive plans" after mobilizing hundreds of thousands of reserves and several days of combat.

In Gaza, at least 3,000 people have been killed, including about 1,000 women and 1,000 children, according to the Palestinian Ministry of Health.

Another 12,500 people in Gaza have been injured.

French President Emmanuel Macron said there's been contact with Hamas via "several friendly intermediary powers" to try and negotiate the release of hostages.

Turkey's Minister of Foreign Affairs, Hakan Fidan, said it's launched talks with Hamas' political wing as it efforts the release of foreigners, civilians and children.

In remarks with Israeli Prime Minister Benjamin Netanyahu, Secretary of State Antony Blinken said at least 25 Americans had now been confirmed dead in Israel.

Netanyahu described some of the atrocities committed by Hamas.

"Hamas has shown itself to be an enemy of civilization: The massacring of young people in an outdoor music festival, the butchering of entire families, the murder of parents in front of their children and the murder of children in front of their parents. The burning of people alive, the beheadings," he said.

"Hamas should be treated exactly the way ISIS was treated," he said. "They shouldn't be spit out from the community of nations. No leader should meet them. No country should harbor them. And those that do should be sanctioned."

Blinken said he was speaking not only as an American diplomat, but as a Jew, a husband and a father of young children, saying it was impossible for him to look at photos of families killed "and not think of my own children."

"The same time that we've been shocked by the depravity of Hamas, we've also been inspired by the bravery of Israel citizens," he said. "The grandfather who drove over an hour to a kibbutz under siege, armed only with a pistol and rescued his kids and grandkids. The mother who died shielding her teenage son with her body, giving her life to save his -- giving him life for a second time."

Blinken remarked that some U.S. aid had already been delivered, and that more military aid was on his way. He said the administration would work with Congress to meet Israel’s additional needs and that bipartisan support for Israel was "overwhelming."

The secretary called on world leaders to condemn Hamas.

Blinken also reiterated that Israel "has the right -- indeed, the obligation -- to defend itself" but that it was "so important to take every possible precaution to avoid harming civilians."

-ABC News' Shannon Crawford