A Syrian rights group says rebels have retreated from villages around a northwestern town after days of shelling by Syrian troops.

The Britain-based Syrian Observatory for Human Rights said Wednesday the rebels had pulled out from areas near al-Haffeh in Latakia province, where government forces have been carrying out attacks with helicopters and tanks.

The U.S. State Department had warned on Monday that reports from inside Syria indicate the forces of President Bashar al-Assad were preparing a massacre in al-Haffeh in revenge for its support of a 15-month opposition uprising.

The Syrian foreign ministry issued a statement through state media late Tuesday denouncing the U.S. allegation, calling it “blatant interference” in Syrian affairs and saying it “coincided” with an increase in attacks by rebels. Damascus accused Washington of continuing support for “terrorists” whom Syrian officials say are driving the revolt.

The Observatory says Tuesday's fighting killed at least 64 people, most of them in government bombardments of rebellious areas such as Aleppo and Deir el-Zour.

U.N. peacekeeping chief Herve Ladsous said Tuesday that Syria has seen a “massive increase in the level of violence” and is now in an all-out civil war. He said the Syrian government has lost several areas to the opposition and is trying to retake them using tanks, artillery and attack helicopters.

U.N. spokesman Martin Nesirky later backed away from Ladsous' reference to a civil war, telling reporters “it is not for the United Nations to designate in that way.” But, Nesirky said there has been “a qualitative shift and intensification” of the violence in Syria.

In another development, U.S. Secretary of State Hillary Clinton said that Washington is concerned about reports that Russia is sending attack helicopters to the Assad government, a longtime Russian ally.

Speaking at the Brookings Institution in Washington, she also rejected Russia's claim that its arms shipments to Syria are unrelated to the uprising, calling it “patently untrue.” She said the United States has urged Russia to stop those arms transfers.