UN Secretary-General Ban Ki-moon has expressed concern that violence from the 14-month conflict in Syria could spread to neighbouring Lebanon.
He reiterated his fear that the Syrian violence may erupt into a full-scale civil war.
Following a meeting between Mr Ban and new French President Francois Hollande on the sidelines of the NATO summit in Chicago, the UN press office wrote that Mr Ban said the world is at "a pivotal moment in the search for a peaceful settlement to the crisis."
It said he was "extremely troubled about the risk of an all-out civil war [in Syria] and was concerned about the outbreak of related violence in Lebanon," the UN statement said.
At least two people were killed in heavy fighting between rival Sunni Muslim gunmen in Beirut this morning, medical and security sources said, in the latest violence fuelled by tensions over the uprising against Syrian President Bashar al-Assad.
UN special co-ordinator for Lebanon Derek Plumbly also voiced concern about the conflict and called on all parties to stop fighting.
"Differences must be addressed through dialogue, not resort to violence," he said in a statement.
UN spokesman Farhan Haq told reporters in New York that Mr Plumbly was in contact with all parties in Lebanon's government, which would include the pro-Syrian Shia Muslim militant movement Hezbollah.
Many of Lebanon's Sunni Muslims sympathise with the Sunni-led uprising in Syria against Mr Assad, whose father sent forces into Lebanon during its 1975-1990 civil war.
The Syrian army finally pulled out in 2005 under international pressure.
Elsewhere, Syrian rebels said police opened fire and killed two people today when a crowd turned out to greet a team of UN ceasefire monitors in the eastern province of Deir al-Zor.
A United Nations spokeswoman said the team had heard gunfire but said they had not seen any casualties.
Reports from Syria cannot be independently verified as state authorities have barred international journalists and rights groups.
