Hezbollah said it launched Katyusha rockets at a north Israel intelligence base it blamed for targeted killings, claiming its first attack after Israel launched a deadly strike on the group's south Beirut stronghold, a day after the group's leader Sayyed Hassan Nasrallah vowed to retaliate against Israel for the device blasts carnage.

The Iran-backed group said its fighters had targeted "the main intelligence headquarters in the northern region responsible for assassinations... with volleys of Katyusha rockets", adding the attack was "in response to the Israeli enemy's attacks" on south Lebanon.

Israeli Defense Minister Yoav Gallant has said Hezbollah would “pay an increasing price” as Israel claims to be seeking to make conditions near its border with Lebanon safe enough for residents to return.

"Hezbollah is feeling chased and the sequence of our military operations will continue," Gallant added, in a possible hint that Israel was behind the deadly pager and walkie-talkie blasts that rocked Lebanon on Tuesday and Wednesday leaving 37 people dead and around 3,000 injured, many of them severely.

Bulgarian authorities said on Friday a company based in Sofia had nothing to do with the delivery of exploding communications devices to Hezbollah.

Hundreds of pagers and walkie-talkies detonated across Lebanon on Tuesday and Wednesday, killing at least 37 people, wounding nearly 3,000 and generating panic.

Israel's army announced new strikes on Hezbollah targets in Lebanon on Thursday, as warning sirens rang out in northern Israel, indicating possible incoming fire.

Hezbollah chief Sayyed Hassan Nasrallah said Thursday that Israel will receive "just punishment" for the communications device explosions that killed 37 people and wounded nearly 3,000 over two days.

Israel will face "tough retribution and just punishment, where it expects it and where it does not," Nasrallah said in his first speech since the blasts, adding that he would not give further details of the place, timing or nature of Hezbollah's response.

U.S. Defense Secretary Lloyd Austin announced Thursday that he spoke Wednesday with Israeli Minister of Defense Yoav Gallant to “review regional security developments and reiterate unwavering U.S. support for Israel in the face of threats from Iran, Lebanese Hezbollah, and Iran’s other regional partners.”

French President Emmanuel Macron has held phone talks with Speaker Nabih Berri and caretaker Prime Minister Najib Mikati over the deadly explosions that rocked Lebanon on Tuesday and Wednesday.

The Israeli military said on Thursday it struck six Hezbollah "terrorist infrastructure sites" and a weapons storage facility in southern Lebanon overnight, as fears grew of a full-blown war.

The air force "struck Hezbollah terrorist infrastructure sites in the areas of Chihine, Taybeh, Blida, Mays al-Jabal, Aitaroun and Kfarkela in southern Lebanon, as well as a Hezbollah weapons storage facility in the area of Khiam in southern Lebanon," a military statement said.

Sayyed Hashem Safieddine, the head of Hezbollah’s Executive Council, said Wednesday that the group will respond to Tuesday's pager explosion attack with “special punishment."

The group is in a “new confrontation with the enemy," Safieddine said.

Israeli forces in the northern region are continuing their offensive and defensive exercises, the Israeli army said on Wednesday.