Syrian army troops backed by pro-government militia members have entered rebel-held neighborhoods of the central city of Homs after laying siege to the districts for nearly two years.

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

Activists on the ground and the Britain-based pro-opposition monitoring group Syrian Observatory for Human Rights on Tuesday both confirmed the operation.

"The Syrian army and the National Defense Forces have achieved key successes in the Old City of Homs," Syrian state television said.

It said troops were advancing in several besieged neighborhoods in the area, and had "killed a number of terrorists," a reference to rebel forces.

"They have entered into one area, Wadi al-Sayeh, which lies between Juret al-Shiyah and the Old City," said Abu Bilal, an activist trapped inside the blockade, told the AFP news agency.

Forces loyal to Syrian President Bashar al-Assad used heavy machine-gun fire, tank fire and aircraft shelling to pound rebels holed up in the Old City, said an activist who uses the name Abu Bilal. He said Tuesday was the heaviest day of fighting Homs had experienced in months.