An Israeli airstrike in Beirut on Tuesday killed Saleh al-Arouri, the deputy chief of Palestinian militant group Hamas, Lebanese authorities confirmed. The attack also claimed the lives of at least five other Hamas members.

Al-Arouri, considered one of Hamas’s main military strategists, was killed when an Israeli drone struck a residential building in Beirut’s southern suburbs, a stronghold of the group’s ally Hezbollah. This marks the first Israeli attack on the Lebanese capital since the latest war erupted between Israel and Hamas in Gaza three months ago.

The Israeli military declined to comment on the strike that assassinated the high-ranking Hamas leader. Al-Arouri spent nearly 20 years in Israeli jails before being freed in 2010 on condition that he go into exile.

Hamas spokesperson confirmed his death on the group’s TV network, stating he was killed in a “treacherous Zionist strike.” Officials said two other members of Hamas’s armed wing died in the targeted bombing.

Lebanese authorities reported that the Israeli drone attack damaged two floors of the building and a vehicle. Debris from the blast spread hundreds of feet into the busy neighborhood.

Prime Minister Najib Mikati condemned the strike, saying it aims to drag Lebanon further into the ongoing Israel-Hamas conflict. Palestinian group Islamic Jihad also denounced the “assassination” of Al-Aruri and his comrades.

Considered the mastermind of multiple attacks on Israel, Al-Arouri was elected as Hamas’s deputy chief in 2017. His killing is seen as a major escalation amid cross-border tensions between Israel and Lebanon’s Hezbollah movement.

Hamas has vowed its “brave resistance” will continue undeterred by the loss of its high-profile deputy leader.