BEIRUT, Lebanon — Syrian government forces backed by Lebanese fighters from the militant group Hezbollah pushed Sunday into parts of Qusayr, a strategic city long held by rebels, according to an antigovernment activist and pro-government news channels. If the advance holds, it would be a serious setback for opponents of President Bashar al-Assad.

Both sides called it one of the war’s most intense ground battles. The fight seemed likely to inflame regional tensions as Hezbollah plunges more deeply into the conflict in Syria, increasing fears of a regional conflagration.

The Syrian military hammered Qusayr, on the Lebanon border, with airstrikes and artillery, killing at least 52 people and wounding hundreds as civilians cowered, unable to flee the city, activists said. By day’s end about 60 percent of the city, including the municipal office building, was under the army’s control for the first time in months, one activist said. Residents said rebels kept fighting into the night in Qusayr, killing a number of Hezbollah and government fighters.

Syrian state television said the army had “tightened the noose on the terrorists,” the government’s term for its armed opponents, by attacking from several directions. State news media said the army had “restored security and stability” to most of the city, killing many rebel fighters and capturing others.