UN Secretary General Ban Ki-moon warned Monday that the search for a peaceful end to the unrest in Syria was at a "pivotal moment," his spokesman said on the sidelines of a NATO summit in Chicago.

"The secretary general said we were at a pivotal moment in the search for a peaceful settlement to the crisis and that he remained extremely troubled about the risk of an all-out civil war," the spokesman said in a statement.

Ban, who met with new French President Francois Hollande in Chicago, was also "concerned about the outbreak of related violence in Lebanon," the statement added.

Street battles between pro- and anti-Syrian groups in Beirut overnight left two people dead, a security official said Monday, sparking concerns the conflict in Syria is increasingly spilling into Lebanon.

NATO Secretary General Anders Fogh Rasmussen voiced concern about the relentless violence in Syria, but ruled out any military intervention against the regime of President Bashar al-Assad.