Lebanese PM Fouad Siniora has accused Israel of a "naked violation" of the five-day-old ceasefire, after a raid by Israeli commandos deep inside Lebanon.
The commandos were airlifted from a cornfield, witnesses say
The raid, in the eastern Bekaa Valley, left one Israeli dead and two injured.
Israel said it was trying to disrupt the movement of weapons from Iran and Syria to Hezbollah, and insisted the ceasefire was still intact.
But Lebanon's defence minister said he might suspend his army's deployment in the south if the UN failed to respond.
The Israeli raid centred on the village of Bodai, west of the city of Baalbek, some 100km (60 miles) north of the Israeli border.
It is the first incident of its kind since the ceasefire came into effect.
Lebanese Defence Minister Elias al-Murr said that if the UN failed to give "clear answers" in response to the raid, "I might be forced to ask the cabinet early next week to halt the army deployment in the south".
A visiting UN envoy, Terje Roed Larsen, told the BBC that he would not pre-judge the raid, but described it as unhelpful to the peace process.
Apparent remnants of the raid were found at the scene
Israeli helicopters are believed to have dropped off the commandos and two vehicles during the night.
The Israelis seem to have met more resistance than they expected, with one local fighter describing a gun battle lasting more than two hours, says the BBC's Jon Leyne at the scene.
Two helicopters landed in a cornfield and took away the soldiers, while Israeli fighter jets circled overhead, witnesses say.
There is speculation locally that the Israelis may have been trying to capture a senior Hezbollah figure who lives in the village, our correspondent adds.
Lebanese sources earlier told Reuters agency that three militants died in the incident.
In Beirut, the raid prompted an angry response from Mr Siniora.
"It is a naked violation of the cessation of hostilities declared by the Security Council," he told reporters.
He said a complaint had been made to visiting UN envoys about the operation.
But Israel insisted it had not breached the ceasefire.
"We had specific information of arms transfers taking place and we acted to prevent that violation, so that violation is not from the Israeli side - we were responding to a violation of the resolution by Hezbollah," said Israeli spokesman Mark Regev.
Israel has said it will continue to carry out such actions until an expanded international military force is in place to prevent Hezbollah's rearmament.
The resolution which stopped the conflict said Israel should end all offensive military action and Hezbollah should end all attacks.
It also called for a halt to all unauthorised arms deliveries to Lebanon.
Fifty French troops arrived in the Lebanese port of Naqoura on Saturday, the first soldiers to bolster the UN peacekeeping force.
They are among 200 extra troops promised by France, as the UN struggles to build its expanded force.
The UN wants 3,500 troops on the ground by the end of August, to be increased later to 15,000. It says it is disappointed with the French contribution and wants other European nations to offer more help too.