As warning sirens wailed across southern and central Israel, including in Jerusalem, Israel’s military said it was on a war footing and the defense minister said the Iran-backed Hamas had made a “grave mistake” by declaring war on Israel.

The Israeli military said it had launched air strikes into Gaza, where witnesses reported hearing heavy explosions.

The attack marked an unprecedented infiltration by an unknown number of Hamas gunmen into Israel from Gaza, and one of the most serious escalations in the Israeli-Palestinian conflict in years. At least two Israelis were killed, Israel public broadcaster Kan reported. Medical officials said there were dozens of wounded.

Israeli media reported gunbattles between bands of Palestinian fighters and security forces in towns in southern Israel. Israel’s police chief said there “21 active scenes” in southern Israel, indicating the extent of the attack.

In Gaza, people rushed to buy supplies in anticipation of days of conflict ahead. Some evacuated their homes and headed for shelters.

Hamas military commander Mohammad Deif announced the start of the operation in a broadcast on Hamas media, calling on Palestinians everywhere to fight.

“This is the day of the greatest battle to end the last occupation on earth,” he said, adding that 5,000 rockets had been launched.

The last major flare-up between Israel and Hamas was a 10-day war in 2021.