Three United Nations observers and a translator were wounded on Saturday when a shell exploded near them as they were carrying out a foot patrol in south Lebanon, the UN peacekeeping mission said, adding it was still investigating the origin of the blast.

The UN peacekeeping mission known as UNIFIL, as well as unarmed technical observers known as UNTSO, are stationed in southern Lebanon to monitor hostilities along the demarcation line between Lebanon and Israel, known as the Blue Line.

Lebanese armed group Hezbollah has been trading fire with the Israeli military across the Blue Line since October in parallel with the war in Gaza.

UNIFIL said in a statement on Saturday that the targeting of peacekeepers is "unacceptable" and that the wounded staff had been evacuated for treatment.

Two security sources had earlier told Reuters the observers were wounded in an Israeli strike outside the border town of Rmeish.

The Israeli military denied involvement in the incident. "Contrary to the reports, the IDF did not strike a UNIFIL vehicle in the area of Rmeish this morning," the military said in a statement.

Lebanon's caretaker Prime Minister Najib Mikati spoke with UNIFIL commander Aroldo Lozaro, condemning the "targeting" and wounding of UN staff in southern Lebanon, according to a statement from Mikati's office.

The mayor of Rmeish, Milad Alam, told Reuters that he had spoken with the Lebanese translator and confirmed his condition was stable.

"From Rmeish, we heard a blast and then saw a UNIFIL car zipping by. The foreign observers were taken to hospitals in Tyre and Beirut by helicopter and car," Milad said, without providing details on their condition.

One of the observers was a Norwegian citizen, who was lightly injured, the Nordic country's defence ministry told Reuters. Lebanon's National News Agency said the other two wounded observers were Chilean and Australian.

Israel's shelling of Lebanon has killed nearly 270 Hezbollah fighters, but has also killed around 50 civilians - including children, medics and journalists - and hit both UNIFIL and the Lebanese army.

UNIFIL last month said that the Israeli military violated international law by firing on a group of clearly identifiable journalists, killing Reuters reporter Issam Abdallah.

The UN's Special Coordinator for Lebanon, Joanna Wronecka, said in a statement that she was "saddened" to learn of the injuries and that the incident served as "another reminder of the urgent need to return to a cessation of hostilities across the Blue Line".

The US and other countries have sought to secure a diplomatic resolution to the exchanges of fire between Hezbollah and Israel. Hezbollah said it will not halt fire before a ceasefire is implemented in Gaza.