The UNIFIL force said the incident on Friday was a "serious development", and that the security of UN personnel and property must be guaranteed.
France summoned Israel's ambassador. Russia said it was "outraged" and demanded that Israel refrain from "hostile actions" against the peacekeepers.
Lebanese army soldiers have been deployed to the Lebanese side of the Israeli border in the south. (AP PHOTO)
UN Secretary-General Antonio Guterres had already condemned the attacks on UN personnel.
The conflict between Israel and Hezbollah erupted one year ago when the Iranian-backed group began launching rockets at northern Israel in support of the Palestinian militant group Hamas, at the start of the Gaza war.
It has intensified in recent weeks, with Israel bombing southern Lebanon, Beirut's southern suburbs and the Bekaa Valley, killing many of Hezbollah's top leaders, and sending ground troops across the border. Hezbollah for its part has fired rockets deeper into Israel.
Beirut is still reeling from Israel's latest hit and the war shows no sign of abating. (AP PHOTO)
Overnight, 22 people were killed and 139 wounded in a strike in the heart of Lebanon's capital Beirut, caretaker prime minister Najib Makati said. Lebanon's army said two soldiers had been killed and three others wounded when Israeli forces attacked one of its military posts in Kafra in the south.
Israel says its campaign in Lebanon aims to make northern Israel safe for tens of thousands of people forced to leave over the last year by Hezbollah rocket fire.
Its expanded operation has displaced more than 1.2 million people, according to the Lebanese government, which says more than 2100 people have been killed and more than 10,000 wounded in over a year of fighting. The toll does not distinguish between civilians and combatants but includes scores of women and children.
Hezbollah rockets have killed at least 54 people in Israel, more than half civilians, according to Israeli authorities.
In a video released on Friday, the chief of staff of the Israeli military, Lieutenant General Herzi Halevi, said it would not stop its campaign "until we ensure that we can safely return the residents not just now, but with a future outlook".
Israel's Iron Dome is doing a good job of intercepting Hezbollah strikes. (EPA PHOTO)
The watchtower that came under Israeli fire on Friday is located at UNIFIL's main base in Naqoura. UNIFIL said an Israeli bulldozer had also knocked over barriers at UN positions near the Blue Line denoting the frontier between Lebanon and Israel, while tanks had moved into the vicinity.
The Israeli military on Friday expressed "deep concern" and said two peacekeepers had been injured by Israeli fire as it was engaging Hezbollah. It said they had been warned hours earlier to take shelter, however. The United Nations said both were from Sri Lanka.
Two Indonesian UN peacekeepers were injured on Thursday after falling from a watchtower after Israeli tank fire, after which Israel said its troops had opened fire nearby, and that Hezbollah fighters operated from areas near UNIFIL posts.
UNIFIL has more than 10,000 personnel, with Italy, France, Malaysia, Indonesia and India among the biggest contributors.
Thursday night's air strike was the third on the centre of the Lebanese capital since Israel went on the offensive on September 23.
Security sources said the target was senior Hezbollah official Wafiq Safa, and that he had survived.
A Hezbollah politician visiting the site on Friday said no senior Hezbollah officials had been present at the time.
In northern Israel, a Thai worker was killed as a result of fallen munition, likely fired from Lebanon, the Israeli military said. It also said the Israeli airforce had killed a Hezbollah commander responsible for attacks with anti-tank missiles into the area of Ramot Naftali in northern Israel.
Hezbollah issued no immediate comment on that claim.
The Middle East remains on high alert for further escalation, awaiting Israel's response to an Iranian missile barrage on October 1.
