The UN's head of peacekeeping has told reporters that Syria is now in a state of civil war and the government has lost control of "large chunks" of cities.
It is the first time a UN official has formally voiced that view.
His comments came as UN monitors in Syria were fired on as they tried to reach the besieged town of Haffa.
Meanwhile, US Secretary of State Hillary Clinton accused Russia of supplying attack helicopters to the Syrian government.
She said the move would "escalate the conflict quite dramatically".
Russia maintains its arms shipments to Syria are unrelated to the conflict there.
Syria blames the violence on foreign-backed armed terrorist gangs.
Meanwhile, Kofi Annan has called for another international conference on Syria, but no date or list of participants has yet been announced.