Early on Thursday, Israel launched airstrikes on central Beirut, killing at least six people, following its military’s deadliest day on the Lebanese front in a year of clashes with Hezbollah, backed by Iran. Witnesses reported hearing a powerful explosion, and videos shared online showed thick plumes of black smoke rising from the area.

The attack targeted a medical center, but it was unclear whether anyone inside was killed. The attack was the closest Israel has come to a central downtown district of the city since 2006. Hezbollah leader Hassan Nasrallah is based in the nearby suburb of Dahieh, which has been hit by several air strikes this week.

An Israeli spokesman said the military had conducted a precise air strike, targeting a “large and dangerous Hezbollah weapons storehouse in the vicinity of Hezbollah headquarters,” and adding that it had been destroyed. A security source confirmed the target and said Hezbollah had not been warned but that a warning had been issued during the operation.

“As a result of this precise operation, the enemy suffered heavy casualties and damage to their positions,” the spokesman added. “Israel will continue to act against Hezbollah.”

It was not immediately clear what the targets were, but Israel has been targeting buildings occupied by Hezbollah in central Beirut. Hezbollah says it will respond with “retaliatory strikes.”

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Residents of the Cola neighborhood told NBC News that they rushed to help clear the rubble from their homes, bringing in broomsticks and even their bare hands. They said that they had been terrified by the bombing, which came just hours after Israel pounded Hezbollah targets in southern Lebanon.

The strikes came a day after Hezbollah suffered its deadliest day of rocket fire on Israel in nearly a decade, but the Iron Dome missile defense system intercepted most of those shots. The attacks also followed the assassination of Hezbollah political official Nabil Kaouk. The Iran-linked group has not yet announced his replacement.

Earlier, Israeli warplanes struck Hezbollah sites in southern Lebanon that the army said were set to fire at Israel in one of its heaviest barrages of attacks on the country in nearly a month. The IDF said that it destroyed more than 100 Hezbollah rocket launchers and other structures.

Lebanon’s government has urged Hezbollah to disarm, but the militia has dismissed the appeal. “Israel’s hegemonic policy will not affect Hezbollah’s decision to remain in Lebanon and fight for the liberation of its land,” the interior ministry said.

The heaviest Israel air strikes on Hezbollah in recent years have hit the southern suburbs of the capital, where the Iranian-backed Shi’ite militia keeps its rocket arsenal and training facilities. But this was the first time Israel had targeted a central area of Beirut itself, and it sent a shockwave through the city.

The heaviest of Israel’s strikes targeted a building in the district of Bachoura near parliament.