UN monitors in Syria reported fiery devastation, the smell of death, vacated homes, looted stores, and vestiges of heavy weapons Thursday during a visit to what had been a Sunni-populated village besieged for days by Syrian forces and progovernment militiamen who said they had cleansed it of rebel fighters.

In a preliminary report on their visit to the village, Haffa in northwestern Syria, a spokeswoman for the monitors said it appeared to be deserted, except for “pockets in the town where fighting is still ongoing.’’ Antigovernment activists reported Wednesday that Haffa’s residents had fled in the face of relentless attacks by the Syrian military.

The siege of Haffa became a focal point of the Syrian conflict this week because of fears expressed by UN and Western officials that its residents were vulnerable to a massacre. Those fears were elevated following mass killings in other Sunni-populated locales over the past few weeks suggesting that the uprising against President Bashar Assad, which began 16 months ago as a peaceful political protest, has now become a sectarian civil war pitting his minority Alawite sect against the majority Sunni populace and other groups.

Anti-Assad activists also reported the extensive use of Russian-made helicopter gunships in the siege of Haffa and attacks in the nearby port of Latakia, a relatively new tactic in Assad’s campaign to crush the uprising and a possible reflection of rebel success in damaging his army’s fleet of Russian tanks. The helicopters also subjected Russia, Assad’s principal backer, to renewed Western criticism as an abettor of his repression. Russia has insisted that it takes no side in the conflict.

The monitors, who are unarmed, were blocked Tuesday from visiting Haffa by a mob of angry civilians, apparently from surrounding villages populated by Alawites.

The Syrian Foreign Ministry announced 24 hours later that the monitors were welcome to visit Haffa, which the ministry said had been rescued from the clutches of armed terrorist groups - the government term for opponents of Assad’s ruling Ba’ath Party.

“The town appeared deserted,’’ Sausan Ghosheh, the spokeswoman for the UN monitor mission in Syria, wrote in the preliminary report. “Most government institutions, including the post office, were set on fire from inside. Archives were burnt, stores were looted and set on fire, residential homes appeared rummaged, and the doors were open.’’

Ghosheh wrote that the local Ba’ath Party headquarters had been shelled and “appeared to be the site of heavy fighting.’’

“Remnants of heavy weapons and a range of caliber arms were found in the town,’’ she wrote. “Cars, both civilian and security, were also set on fire and damaged.’’

She also wrote that a “strong stench of dead bodies was in the air,’’ but there was no information on the number of casualties.

In Moscow on Thursday, Syria’s ambassador to Russia, Riad Haddad, said at a news conference that Russia was supplying his government only with antiaircraft weapons, not attack helicopters. He was echoing statements made this week by top Russian officials in response to US accusations that Russia had risked deepening the Syrian conflict through its military support of Assad.

Haddad also rejected descriptions of the conflict as a civil war, made this week by UN and French officials.

Diplomacy aimed at halting the conflict has faltered despite rising violence. Kofi Annan, the special envoy of the UN and Arab League, whose peace plan placing the monitors in Syria is widely considered near failure, has sought to convene a meeting of influential countries to press all sides in the conflict to honor the cease-fire.

But there has been no word on when and where such a meeting would take place.

In the central Syrian town of Rastan north of Homs, where rebels have defied persistent military efforts to rout them, an activist reached through Skype said the situation had deteriorated in three consecutive days of bombing from land and air.

The activist, who identified himself as Morhaf al Zoaby, said the Syrian forces were using tanks, helicopters, cluster bombs, and rockets emitting an unidentified gray-black gas, killing at least four people. It was impossible to verify his account.