An Israeli airstrike killed four people including a Hezbollah official in Beirut’s southern suburbs on Tuesday, a Lebanese security source said, further testing a shaky ceasefire between Israel and Iran-backed Hezbollah.

The Israeli military said the official – Hassan Bdeir – was a member of a Hezbollah unit and Iran’s Quds Force, and he had assisted the Palestinian group Hamas in planning a “significant and imminent terror attack against Israeli civilians,” Reuters reported.

The Lebanese security source said the target was a Hezbollah figure whose responsibilities included the Palestinian file. The Lebanese health ministry said the strike killed four people – including a woman – and wounded seven others.

It marked Israel’s second airstrike in the Hezbollah-controlled suburb of Beirut in five days, adding to strains on the U.S.-brokered ceasefire that ended last year’s devastating conflict.

The attacks on Beirut’s southern suburbs have resumed at a time of broader escalation in the region, with Israel having restarted Gaza strikes after a two-month truce and the United States hitting the Iran-aligned Houthis of Yemen in a bid to get them to stop attacking Red Sea shipping.

Hezbollah lawmaker Ibrahim Moussawi said the Israeli attack amounted to “a major and severe aggression that has escalated the situation to an entirely different level”.

Speaking in a televised statement after visiting the building that was struck, he called on the Lebanese state to “activate the highest level of diplomacy to find solutions”.

Israeli Foreign Minister Gideon Saar said the eliminated Hezbollah operative posed “a real and immediate threat”. “We expect Lebanon to take action to uproot terrorist organizations acting within its borders against Israel,” he said.

Israel dealt severe blows to Hezbollah in the war, killing thousands of its fighters, destroying much of it arsenal and eliminating its top leadership including Hassan Nasrallah.

Hezbollah has denied any role in recent rocket attacks from Lebanon towards Israel, including one that prompted Israel to carry out an airstrike on the southern suburbs last Friday.

Tuesday’s strike in the early hours appeared to have damaged the upper three floors of a building, a Reuters reporter at the scene said, with the balconies of those floors blown out.

The glass on the floors below was intact, indicating a targeted strike. Ambulances were at the scene as families fled to other parts of Beirut.

There was no advance warning, in contrast to the attack on Friday when the Israeli military announced which building it intended to hit and ordered residents to leave the area.

Lebanese President Joseph Aoun condemned the latest airstrike, calling it a “dangerous warning” that signals premeditated intentions against Lebanon, which would intensify diplomatic outreach and mobilise international allies.

Lebanese Prime Minister Nawaf Salam said the strike was a flagrant breach of a U.N. Security Council Resolution upon which the ceasefire was based, and the ceasefire arrangement.

U.S. BACKS ISRAEL

The ceasefire agreement demanded that southern Lebanon be free of Hezbollah fighters and weapons, that Lebanese troops deploy into the area, and that Israeli troops withdraw.

But each side accuses the other of failing to implement the terms fully. Israel says Hezbollah still has infrastructure in the south, while Lebanon and Hezbollah say Israel is occupying Lebanese soil by not withdrawing from five hilltop positions.

The U.S. State Department said that Israel was defending itself from rocket attacks that came from Lebanon and that Washington blamed “terrorists” for the resumption of hostilities.

“Hostilities have resumed because terrorists launched rockets into Israel from Lebanon,” a State Department spokesperson said in an email, responding to a question from Reuters seeking reaction to Tuesday’s airstrike. Washington supported Israel’s response, the spokesperson said.

The Israel-Hezbollah conflict was ignited when Hezbollah opened fire in support of Hamas at the start of the Gaza war. It escalated in September when Israel went on the offensive, declaring the aim of securing the return home of tens of thousands of people evacuated from homes in the north.

The war uprooted more than a million people and killed at least 3,768 people in Lebanon, according to a Lebanese health ministry toll from November. Dozens more have been reported killed by Israeli fire since the ceasefire.

Lebanon’s figures do not distinguish between civilians and fighters.

During the war, Hezbollah strikes killed 45 civilians in northern Israel and the Israeli-occupied Golan Heights. At least 73 Israeli soldiers were killed in northern Israel, the Golan Heights, and in combat in southern Lebanon, according to Israeli authorities.