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The United Nations said four of its military observers were wounded Saturday when a shell exploded near them in southern Lebanon, where Israel and the Hezbollah movement have traded regular cross-borderÌýfire since Israel's military onslaught in Gaza began last October.

Lebanon's official National News Agency said Israel was behind the explosion, though the United Nations Truce Supervision Organization (UNTSO) has not yet identified the source of the shelling.

"This morning four UNTSO military observers on a foot patrol along the Blue Line (demarcating the border with Israel) were injured when a shell exploded near their location," UNTSO said.

"The targeting of peacekeepers is unacceptable," it said in a statement, adding that they were taken to hospital for treatment and an investigation launched into the origin of the explosion.

"All actors have a responsibility under international humanitarian law to avoid targeting non-combatants, including peacekeepers, journalists, medical personnel, and civilians.

"We repeat our call for all actors to cease the current heavy exchanges of fire before more people are unnecessarily hurt," UNTSO said.

°Õ³ó±ðÌýNational News Agency said "enemy" aircraft raided the Rmeish area in southern Lebanon where the UNTSO observers were wounded.

Lebanese Prime Minister Najib Mikati in a statement condemned what he called a "dangerous incident".

Israel has killed at least 347 people, mostly Hezbollah fighters, in Lebanon since October but also including at least 68 civilians, including a Reuters journalist. The fighting has displaced tens of thousands of people in southern Lebanon.

An uptick in deadly exchanges in recent days has sparked further concerns of an all-out conflict between Israel and Hezbollah, who last fought a war in 2006.

UNTSO was set up in 1948 following the creation of Israel and the uprooting of thousands of Palestinian refugees in the Nakba as a peacekeeping mission to monitor ceasefires and armistice agreements between Israel and Arab countries such as Lebanon.

It also assists other UN peacekeeping operations in the region, including the UNIFIL peacekeeping force tasked with monitoring the Israeli-Lebanese border area.