BEIRUT — Syrian government forces backed by Lebanese fighters from the militant group Hezbollah pushed Sunday into parts of a strategic city long held by rebels, according to both an antigovernment activist and progovernment news channels.
If the advance holds, it would be a serious setback for opponents of President Bashar Assad and further inflame regional tensions.
The Syrian military hammered the city, Qusair, on the border with Lebanon, 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 in army control for the first time in months, activists said.
Syrian state news media said that the army had ‘‘restored security and stability’’ to most of Qusair, killing many fighters and capturing others. 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.
The battle for the city, in heavily contested Homs Province, has deepened the involvement of Hezbollah in the Syrian conflict, raising sectarian tensions and fears of a regional conflagration. The fight is viewed by loyalists and government opponents alike as a turning point that could, in the words of one activist in Qusair, ‘‘decide the fate of the regime and the revolution.’’
‘‘It is one of the hardest days all over Syria,’’ said the activist, who would give only his first name, Tarek, because of security concerns. ‘‘If Qusair is finished, it will be the end of the revolution in Homs.’’
Assad, according to people who have spoken with him, believes that reasserting control in Homs Province is crucial to maintaining control of the string of population centers in western Syria and eventually to military campaigns to retake rebel-held territory in the north and east. Many analysts say it is unlikely that the government will be able to regain control of those areas, but that it could consolidate its hold on the west, leading to a de-facto division of the country.
The battle has brought Hezbollah’s role in Syria to the forefront as the war becomes a regional conflict, pitting Shi’ite-led Iran, the main backer of Assad and Hezbollah, against the Sunni Muslim states and their Western allies that support the uprising.
Tensions have risen in Lebanon as Syrian rebels have shelled Hezbollah-controlled areas. On Sunday, they hit the Lebanese town of Hermel with Grad missiles, activists said.
Activists said Hezbollah’s intervention in Syria would deter the rise of Sunni extremist groups like Al Nusra Front among the rebels.
Lebanese news media and residents of the Bekaa Valley bordering Syria have reported a recent increase in the funerals of Hezbollah fighters who have been fighting near Qusair.
Though many Lebanese Shi’ites support Assad against an uprising in which Sunni extremists are playing an increasing role, there is quiet consternation that the Syrian conflict is growing more bloody and that Hezbollah guerrillas are being sent to battle fellow Arab Muslims in a country where they have many ties, rather than fighting their primary foe, Israel.
Perhaps seeking to address such concerns, Hezbollah, which depends on Assad for its shipments of weapons from Iran, recently acknowledged its military role in Syria more openly. The group’s leader, Hassan Nasrallah, has said the group would not allow Qusair, or the Syrian government, to fall to a rebellion that it views as being used by Israel and the West to their advantage.
For weeks, Hezbollah — which is both Lebanon’s most powerful political party and a militant group listed by the United States government as a terrorist organization — has fought alongside the Syrian military and pro-government militias in villages near Qusair.
The small city, about 100 miles northwest of the Syrian capital, Damascus, is crucial to supply routes for both sides. Qusair is a conduit for rebel supplies and fighters from Lebanon, and it links Damascus to the Mediterranean coast, which is the heartland for Assad’s minority Alawite sect.
The Syrian government appears to be trying to regain as much territory as possible to strengthen its negotiating position while Russia and the United States try to organize peace talks for next month.