Israel says it has killed Hezbollah’s most senior military official, Fu’ad Shukr, in a drone strike in the southern suburbs of Beirut, a populous neighborhood that is also the Iran-backed group’s stronghold.

If true, Shukr would be the most high-ranking Hezbollah official to have been assassinated since 2016 when Mustafa Badreddine, the group’s top commander at the time, was killed in Syria.

Hezbollah has not yet confirmed Shukr’s death. He was a senior military adviser to the group’s Secretary General Hassan Nasrallah, considered his right-hand man, and a member of the jihad council. The US government had put out a reward of $5 million for information about him and his whereabouts.

Whatever the fate of Shukr, Wednesday’s strike is the most serious Israeli escalation since confrontations between Hezbollah and Israel began on October 8.

Israel said it served as retaliation for a deadly strike in the Israeli-occupied town of Majdal Shams in the Golan Heights on Saturday, which Israel blamed on Hezbollah. Hezbollah denied the claim.

The developments have raised the specter of an all-out war between Lebanon and Israel that could extend beyond the borders of both countries.

Nasrallah has repeatedly threatened to hit Tel Aviv in response to any Israeli strike on Beirut, a move that would thrust the two countries, and the region, into uncharted waters.