Syria in civil war, says UN official Herve Ladsous
Syria is now in a state of civil war and the government has lost control of "large chunks" of cities, the UN's head of peacekeeping has told reporters.
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.
Mr Ladsous, UN under-secretary for peacekeeping operations, said the attack on the UN team near Haffa was deliberate.
Asked whether he believed Syria was now in a civil war, Mr Ladsous told a small group of reporters: "Yes, I think we can say that.
"Clearly what is happening is that the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control."
The UN and the US have warned of an alarming escalation in violence in Haffa, amid reports of a build-up of government forces around the town.
UN monitors first tried to reach Haffa on Monday but were denied access.
On Tuesday, government forces gave them permission to pass through the last checkpoint before the town, but the monitors judged the situation to be "unsafe" and turned back, a UN spokeswoman said.
As they were leaving, an angry crowd threw stones and metal bars at the UN team before unknown assailants opened fire, the spokeswoman said.
None of the observers was hurt.
The BBC's Barbara Plett, at the UN in New York, says UN officials have been speaking this week not only about an intensification of government military operations, including firing from helicopters, but also about a dramatic increase in more sophisticated urban attacks by the opposition.
Earlier this month, activists said Syrian government forces killed 108 people in the region of Houla, in Homs province, and 78 people in the village of Qubair, in Hama province.
Syria blames the violence on foreign-backed armed terrorist gangs.
The UN monitors are in Syria to observe the implementation of a peace plan brokered by UN-Arab League envoy Kofi Annan. However, a ceasefire which was supposed to come into effect on 12 April never took hold.
Mr Annan has called for another international conference on Syria, but no date or list of participants has yet been announced.
Activists said Syrian forces fired mortars at protesters in the eastern city of Deir al-Zour, killing at least 10 people.
They also reported clashes in central Homs province.
These claims cannot be verified independently as Syria severely restricts journalists' freedom of movement.