BEIRUT—Smoldering buildings, looted shops, smashed cars and a strong stench of death greeted UN observers who entered the nearly deserted Syrian town of Haffa on Thursday, a day after President Bashar Assad’s forces overran it as part of a major offensive to recover rebel-controlled territories.

The monitors had been trying to get into the town for a week after fears were raised that a brutal assault by regime forces was under way. They found the main hospital burned, state buildings and an office of the ruling Baath party in ruins and a corpse lying in the street.

“A strong stench of dead bodies was in the air,” said Sausan Ghosheh, spokeswoman for the UN observers. She said there was still fighting in some pockets of the mountainous town in the seaside province of Latakia.

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The number of casualties was unclear, Ghosheh said, and it appeared likely that, as in the past, bodies had been removed or buried before the UN mission got in.

The siege of Haffa, a Sunni-populated village, had become a focus of international concern because of fears the uprising against Assad is evolving into a sectarian civil war pitting his minority Alawite sect against the majority Sunnis and other groups. Recent mass killings in other Sunni-populated areas have fed those concerns.

The fighting, now mostly quelled in Haffa, was mirrored in other parts of Syria, where more than 40 civilians and opposition fighters were killed Thursday, according to activists, alongside more than a half-dozen Syrian forces.

From the day’s early hours, Syrian troops bombarded rebel-held areas with tanks, mortars and helicopters in the central town of Rastan, the Damascus suburb of Douma, the central city of Homs and the northern towns of Anadan and Hreitan, near the Turkish border, the activists said.

They said the fighting included clashes in the town of Hamuriya, near Damascus, that killed at least nine men who were allegedly butchered with knives. A video circulated by activists showed a pile of lifeless men, including one who was clearly slashed through the neck.

READ MORE: Syria claims to have retaken rebellious Haffa region after 8 days of clashes

“Slaughter, slaughter!” a person could be heard screaming in the background. Another video showed a man lying in a garden, his arm blown off. There was no way to independently confirm the content of the videos because reporters are not allowed to work freely in Syria.

For more than a week, Syrian troops have been sweeping through villages and towns in Syria’s northern, central, southern and seaside provinces, attacking rebel-held areas and opposition strongholds in what appears to be the largest offensive since an internationally-brokered cease-fire went into effect two months ago. The regime and the opposition have both largely ignored the April 12 truce.

The UN observers’ description of the smoldering ruins they found in Haffa suggested Syrian forces were using intense force to quell rebels. But it also indicated the rebels were determined to smash all symbols of the hated Assad regime, including state institutions.

“Most government institutions, including the post office, were set on fire from inside,” Ghosheh said in a statement. “Archives were burnt, stores were looted and set on fire.”

She said homes were broken into, while the ruling Baath party headquarters was shelled, “and appeared to be the scene of heavy fighting.” The observers also found remnants of heavy weapons scattered through the town; it was not clear who they belonged to. “The town appeared deserted,” she said.

On Tuesday, the unarmed UN monitors were blocked from entering Haffa by a crowd of angry civilians, apparently residents of nearby Alawite villages, who hurled rocks and sticks at the mission’s vehicles. But the Syrian government urged the observers to return after it announced Wednesday that pro-Assad forces had “cleansed” Haffa of “armed terrorist groups” — the regime’s term for rebel fighters.

The UN observers’ visit to Haffa came hours after a suicide bomber detonated a van packed with explosives in a Damascus suburb, wounding 14 people and damaging one of Shiite Islam’s holiest shrines, according to witnesses and Syria’s state-run news agency.

It was not immediately clear whether the bomber intended to target the golden-domed Sayyida Zainab complex or a police station 15 meters away. Believed to house the remains of the granddaughter of Islam’s Prophet Muhammad, the shrine attracts tens of thousands of Shiite pilgrims from around the world.

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UN observers have reported a steep rise in violence and a dangerous shift in tactics by both sides in Syria in recent weeks. Car bombings and suicide bombings have become increasingly common as the 15-month uprising against Assad becomes militarized. Most have targeted security buildings and police buses, symbols of Assad’s regime.

Rami Abdul-Rahman of the Britain-based Syrian Observatory for Human Rights, said it appeared the Syrian regime was trying to implement a kind of scorched earth policy in the central city of Homs, which government forces have been heavily shelling for the past week. He said the use of tanks and attack helicopters to smash residential buildings and city infrastructure indicated they wanted to destroy areas, not just chase out rebels.

In Rastan, a rebel-held town that was heavily bombed Thursday, two rights groups said the dead included Maj. Ahmad Bahbouh, an army defector who headed the town’s opposition military council. Activists said helicopters pounded the town, which has been held by rebels for months.

They said troops also heavily bombed the Damascus suburb of Douma killing at least five people. Abdul-Rahman said Syrian forces seized control of the northern town of Hreitan, where they conducted house-to-house raids and set homes of anti-government activists on fire.

An amateur video from the nearby town of Anadan, showed two babies receiving treatment in a makeshift hospital. One baby, screaming in pain, had part of his left foot blown off and severe head injuries. A third baby was dead and covered with a blanket. Activists said the children were the victims of government shelling. There was no way to verify the claim.

Activists say some 14,000 people have been killed since the uprising against Assad began in March 2011.

Meanwhile, Secretary of State Hillary Rodham Clinton’s accusation that Russia has “dramatically” escalated the crisis by sending attack helicopters to Syria lost some steam Thursday when the State Department acknowledged the helicopters were actually refurbished ones already owned by the Assad regime.

The claim had complicated the Obama administration’s larger goals for Syria and U.S.-Russia relations before a key meeting of the nations’ two leaders.

State Department spokeswoman Victoria Nuland insisted, however, that the nuance meant little, even as she refused to explain why the department didn’t divulge the information earlier.

“Whether they are new or they are refurbished, the concern remains that they will be used for the exact same purpose that the current helicopters in Syria are being used, and that is to kill civilians,” Nuland told reporters in Washington.

“When you look at the Soviet- and Russian-made helicopters that are in use in Syria today, every helicopter that is flying and working is attacking a new civilian location,” she said. “So the concern is when you add three more freshly refurbished helicopters to the fight, that is three more that can be used to kill civilians.”

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