Israel declared war Sunday as it bombarded the Gaza Strip with airstrikes in retaliation for a major surprise attack by Hamas. The declaration came a day after an unprecedented incursion by Hamas fighters who blew through a fortified border fence and gunned down civilians and soldiers in Israeli communities along the Gaza frontier during a major Jewish holiday.

As many as 1,000 Hamas fighters took part, U.S. Secretary of State Andrew Blinken said. The rampage included an assault on a crowded music festival where authorities had removed about 260 bodies by Sunday.

Israel struck back including with airstrikes that flattened a 14-story tower that held Hamas offices. More than 400 people were reported killed in Gaza and at least 700 in Israel.

What does the war declaration mean?

Israel has previously carried out major military campaigns in Lebanon and Gaza that it portrayed as wars, but without a formal declaration.

The declaration gives the green light for Israel to take “significant military steps” against Hamas. It came as the military continued efforts to stamp out the last groups of militants in southern Israel following the attack.

Israel hit more than 800 targets in Gaza by Sunday, its military said. That included airstrikes that leveled much of the town of Beit Hanoun in the enclave’s northeast corner.

Hamas had been using the town as a staging ground for attacks, Israeli Rear Adm. Daniel Hagari told reporters.

There was no immediate word on casualties, and most of the community’s population of tens of thousands likely fled before the bombardment.

The leader of the Palestinian Islamic Jihad, which participated in Saturday’s attack, said it was holding more than 30 Israelis among dozens of captives in Gaza. He said they would not be released until all Palestinian prisoners in Israeli jails are free