Hamas leader Yahya Sinwar was killed by Israeli forces in the Gaza Strip on Thursday, Israeli Foreign Minister Israel Katz said.

He is considered the mastermind of the attack on Israel on October 7, 2023, which resulted in the deaths of 1,200 people, the worst terrorist attack in Israel's history.

“Master assassin Yahya Sinwar, responsible for the October 7 massacre and atrocities, was killed today by IDF soldiers,” Katz said in a statement. “This is a major military and moral achievement for Israel and a victory for the entire free world against the evil axis of extreme Islam led by Iran.”

Israeli authorities said they had been tracking Sinwar for a year and that he had been “hiding behind the civilian population of Gaza, both above ground and underground in Hamas tunnels in the Gaza Strip.” The Israel Defense Forces and Israel Security Service said their operations in recent weeks in the southern Gaza Strip had restricted his freedom of movement and “led to his elimination.”

“Sinwar died while beaten, pursued and on the run – he died not as a commander, but as someone who only cared about himself,” Israeli Defense Minister Yoav Gallant said in a statement. “This is a clear message to all our enemies – the IDF will reach out to anyone who attempts to harm the citizens of Israel or our security forces, and we will bring you to justice.”

The IDF initially said it was “examining the possibility” that the Hamas leader was among three militants killed in an operation in Gaza and was working to confirm the identification through dental images and DNA testing.

Katz said Sinwar's death “creates an opportunity for the immediate release of those abducted and for change that will lead to a new reality in Gaza – without Hamas and without Iranian control.”

Israeli President Isaac Herzog also said in a statement that Israel must “act in every possible way to bring back the 101 hostages” still in the Gaza Strip.

Sinwar, 62, had been Hamas's leader in Gaza since 2017 and took over as head of the group's political office following the assassination of Ismail Haniyeh in Iran in July this year.

President Joe Biden has been briefed on the Israeli investigation into whether Israel killed Sinwar, a senior administration official said.

The Israelis also informed U.S. Defense Department officials, including Secretary Lloyd Austin, of Sinwar's possible death, a U.S. defense official said Thursday, according to a pool report.

In 1989, an Israeli court sentenced Sinwar to four life sentences for his role in the killing of suspected Palestinian informants and conspiring to murder two Israeli soldiers.

Sinwar spent the next 22 years in prison before becoming one of more than 1,000 Palestinian prisoners released in 2011 in exchange for Israeli soldier Gilad Shalit, who had been held hostage by Hamas for five years.

This is a developing story. Please check back for updates.

ABC News' Guy Davies contributed to this report.