Israeli forces fired at a watchtower used by UN peacekeepers in southern Lebanon on Friday, injuring two, a UN source said, the third day in a row peacekeepers have reported Israeli fire at their positions as Israel wages war on Hezbollah.

Renewing his call for a ceasefire, Lebanon's caretaker Prime Minister Najib Mikati said a strike in the heart of Beirut late on Thursday had killed 22 civilians and wounded another 139. The target was a senior Hezbollah official — Wafiq Safa — who survived, according to three security sources.

The conflict between Israel and Hezbollah erupted one year ago when the Iranian-backed group opened fire in support of Palestinian militant group Hamas at the start of the Gaza war.

It has intensified dramatically in recent weeks, with Israel bombing Beirut's southern suburbs, the south and the Bekaa Valley, killing many of Hezbollah's top leaders, and sending ground troops into areas of southern Lebanon, and Hezbollah firing rockets deeper into Israel.

Thursday night's airstrike marked the third in Beirut itself since Israel went on the offensive on September 23.

At a Beirut hospital that received dozens of wounded, a man sat in a chair in a corridor, his knees covered in white bandages, and his face and body covered with wounds.

"The situation ... I don’t even know how to describe it. We received three martyrs ... in pieces,” said Wael al-Jaroush, head of the Makassed hospital's medical department.

"This is the first time we’ve received such a large number of injured from a strike in Beirut."

Among the dead was a family of eight, including three children, who had evacuated from the south, according to a security source.

Safa, the apparent target, performs a political and security role in Hezbollah, suggesting Israel could be widening its targeting of the Hezbollah leadership beyond the military commanders and top echelon it has so far focused on.

In northern Israel, a Thai worker was killed as a result of fallen munition, likely fired from Lebanese territory, the Israeli military said. The Israeli airforce killed a Hezbollah commander responsible for attacks with anti-tank missiles into the area of Ramot Naftali in northern Israel, the military said.