Damascus sent a letter to Lebanese defense officials warning that it may take military action against armed gangs suspected of crossing the Lebanese-Syrian border.
"Armed terrorist groups have infiltrated in large numbers in the past 36 hours from Lebanese territory into Syrian territory," the letter was quoted by Lebanese newspaper The Daily Star as saying.
Damascus warned that violating Syrian sovereignty would hurt "the good brotherly relations between the two countries."
This week marked the seventh anniversary of the Cedar Revolution in Lebanon that diminished Syrian influence over Beirut politics.
The U.N. Security Council said it was concerned about Lebanon's stability given the Syrian civil war.
"(Members) appealed to all Lebanese people to preserve national unity in the face of attempts to undermine the country's stability and stressed the need for all Lebanese parties to respect Lebanon's policy of disassociation and to refrain from any involvement in the Syrian crisis," a Security Council statement read.