CAIRO - The 15-month conflict in Syria has descended into civil war, a United Nations official said on Tuesday, marking the first time the U.N. has classified the bloody conflict as a civil war. It came the same day the United States accused Russia of sending attack helicopters to help the regime of President Bashar al-Assad put down the uprising.

Asked if the conflict is now a civil war, the U.N.'s peacekeeping head said, "Yes, I think we can say that."

"Clearly what is happening is that the government of Syria lost some large chunks of territory in several cities to the opposition and wants to retake control of these areas," Undersecretary General for Peacekeeping Operations Herve Ladsous told the Reuters news agency.

"Now we have confirmed reports of not only the use of tanks and artillery but also attack helicopters," he added. "This is really becoming large scale."

Top U.N. and American officials had repeatedly warned that the increasingly sectarian nature of the conflict had brought Syria to the brink of civil war.

In Washington, Secretary of State Hillary Clinton told an audience at the Brookings Institute on Tuesday that the U.S. is "concerned" about news of attack helicopters being sent from Russia to Syria "which will escalate the conflict quite dramatically."

"We have confronted the Russians about stopping their continued arms shipments to Syria," she said. "They have, from time to time, said that we shouldn't worry - everything they are shipping is unrelated to [the Syrian regime's] actions internally. That's patently untrue."

Russia has been a major backer of Syria, blocking two U.N. resolutions condemning the government's use of force. The U.N. estimates that more than 10,000 Syrians have died since the uprising began last March.

A U.N. report released Monday said children have been maimed, tortured with electricity, sexually assaulted and used as human shields by the Syrian military.

The U.N. has 300 monitors inside Syria who have regularly been blocked from inspecting, mobbed and attacked by both the Syrian military and civilians. On Tuesday, a spokeswoman said observers failed to reach the besieged northwestern town of al-Haffeh where the U.S. has said it fears a "potential massacre."

The U.N. observers "were confronted with angry crowds that surrounded their vehicles" and then "hurled stones and metal rods," spokeswoman Sausan Ghosheh said. As their vehicles retreated, three were fired on. It is unclear by whom.

Video released by the U.N. of a Monday trip to the heavily shelled city of Homs showed smoke rising from the city and widespread destruction.

"Nobody help us. Why?" an unidentified male resident in the clip pleaded. "We are people, not animals."

But despite the continuing violence, Clinton is holding firm to the U.S. policy that arming the opposition is not the answer, that the solution will have to be political.

"There are lots of weapons on the black market, there's money that's available, " said Clinton. "There seems to be an increasing capacity in the opposition both to defend themselves and to take the fight to the Syrian army in an irregular way."

Clinton said that many elements of Syrian society are "terrified" about what comes next after Assad goes, and that is also hindering the political transition. "We haven't had a wholesale departure support or even into exile of Syrian society," said Clinton. "We are approached on a regular basis by different groups within Syrian who are terrified about what comes next." She said that the international community must work with minority groups in Syria, such as Christians and Druids, to "manage a political transition and provide a political transition … that gives some possible reassurance to those who fear what comes next."