DAMASCUS, Syria — Syrian forces tried to seize rebel-held areas of Homs on Tuesday, with fighters weakened by defections and cut supply lines, in some of the fiercest fighting in the central city for months, activists said.

Homs is the last chief stronghold for rebels in central Syria, and the fight to take it underscores how emboldened Syrian forces have methodically taken back opposition-held areas, bolstered by fighters from the Lebanese Shiite group Hezbollah.

Forces loyal to Syrian President Bashar Assad used heavy machine gun fire, tank fire and aircraft shelling to pound rebels holed up in the old city of Homs, said an activist who uses the name Abu Bilal. The assault began five days ago, but Tuesday was the heaviest day of fighting in months, he said.

“They are trying to attack us from a series of axis points,” he said over Skype as heavy shelling could be heard in the background.

Rebels fired mortar shells into government-controlled neighborhoods, killing 15-year-old Tarek Ghrair, a player for the country’s national youth football team, according to the Syrian Football Association’s president.