An explosion in Beirut on Tuesday killed Saleh al-Arouri, a top official with the Palestinian militant group Hamas, and three others, officials with Hamas and the Lebanese group Hezbollah said.

Lebanon’s state-run National News Agency said the blast killed six people and was carried out by an Israeli drone. Israeli officials declined to comment.

If Israel is behind the attack it could mark a major escalation in the Middle East conflict. Hezbollah leader Sayyed Hassan Nasrallah has vowed to retaliate against any Israeli targeting of Palestinian officials in Lebanon. Lebanon’s Prime Minister Najib Mikati said many people were injured in the explosion, which he called “a crime” and said was meant “to drag Lebanon into a new phase of confrontation with Israel.”

Hamas official Bassem Naim confirmed to The Associated Press that al-Arouri was killed in the blast. A Hezbollah official speaking on condition of anonymity in line with regulations also said al-Arouri was killed.

Al-Arouri, one of the founders of Hamas’ military wing, had headed the group’s presence in the West Bank. Israel’s Prime Minister Benjamin Netanyahu had threatened to kill him even before the Hamas-Israel war began on Oct. 7.

The United States government had previously offered a reward of up to $5 million for information on al-Arouri, saying he had “been linked to several terrorist attacks, hijackings, and kidnappings.”

The explosion shook Musharafieh, one of the Lebanese capital’s southern suburbs that are a stronghold of the militant Hezbollah group, an Iran-backed ally of Hamas and one of the world’s most heavily armed non-state military forces.

The explosion came during more than two months of heavy exchanges of fire between Israeli troops and members of Hezbollah along Lebanon’s southern border. Hezbollah fighters and Israeli soldiers have been killed in the fighting there, and Israel has evacuated thousands of civilians from border communities.

The fighting has mainly been concentrated a few miles from the border, but on several occasions Israel’s air force hit Hezbollah targets deeper in Lebanon. Earlier on Tuesday, Hezbollah said its fighters carried out several attacks along the Lebanon-Israel border targeting Israeli military posts.

Israel’s Minister of Defense Benny Gantz said late last month that, if the Hezbollah attacks did not stop along the border, Israel’s military would work to push the armed group back inside Lebanon.