A senior United Nations envoy in Beirut said the UN was trying to establish what had happened in the dawn raid in the Bekaa valley but added that if media reports were true, it would undoubtedly represent a breach of the six-day-old truce.
"We had no independent means to verify...what has happened," envoy Terje Roed-Larsen told Lebanon's LBC television. "But if what has been reported is correct, it is of course a clear violation of the ceasefire."
Israel said the operation, in which commandos were airlifted into the area by helicopter, was defensive and was designed to disrupt weapons supplies to Hizbollah from Syria and Iran.
It denied it had violated the resolution, which allows it to act in self-defence, and accused Hizbollah of doing so by smuggling weapons. Roed-Larsen said that if the guerrilla group was found to have smuggled weapons, it would indeed be in breach of the truce.
About 16 hours after the raid, details of what took place remained unclear.
Lebanese security sources said Israeli helicopters unloaded two vehicles carrying commandos who headed towards an office of a Hizbollah leader, Sheikh Mohammed Yazbek in Bodai, 26km from the Syrian border.
The Israelis were intercepted and withdrew under cover of air strikes, they said. The sources said three Hizbollah guerrillas were killed in a firefight with the Israelis, although Hizbollah said none of its fighters were killed or wounded. Israel said it had suffered one dead and two wounded. Lebanese Prime Minister Fouad Siniora described the operation as "a naked violation of the cessation of hostilities declared by the Security Council".
The official Lebanese news agency said UN secretary-general Kofi Annan spoke to Siniora to assure him the UN would try to prevent another such incident.
Annan also discussed the raid with Israeli Prime Minister Ehud Olmert, who "pointed out the importance of supervision of the Syrian-Lebanese border", an Israeli official said. Syria, like Iran, denies arming Hizbollah. Washington, Israel's chief ally, said it had noted the Jewish state's position.
"The prevention of the re-supply of weapons to Hizbollah by Syria and Iran is a key provision of the UN Security Council resolution 1701," a White House official said.
Lebanon's defence minister said that, if Israeli carried out further, similar operations in Lebanon, he would ask the cabinet to reconsider its decision to deploy Lebanese troops to the south of the country.
That would be a serious blow to a UN plan for southern Lebanon, which envisages a 30,000-strong force in the area made up of Lebanese and UN troops.
Fifty French military engineers disembarked in Naqoura in the south on Saturday, the first reinforcements since the war.
The engineers were among 200 pledged by France, which has disappointed the UN and the US hopes that it would form the backbone of the expanded force to supervise the truce, support the Lebanese army and monitor the withdrawal of Israeli troops.
In his weekly radio address Saturday, the US President George W. Bush said the UN force would help the government in Beirut restore sovereignty and "stop Hizbollah from acting as a state within a state".