Syrian regime forces have repelled a fierce assault by opposition fighters seeking to reopen their only supply route into Aleppo city, killing at least 29 rebels, a monitoring group said on Sunday.



The offensive sought to push government forces back from Castello Road, which leads into the opposition-held eastern half of Aleppo city - now effectively besieged by government troops.



The Syrian Observatory for Human Rights said 29 fighters from the Faylaq al-Sham rebel group and al-Qaeda affiliate al-Nusra Front were killed in fighting or by mines laid by government troops.



The UK-based observatory group said there were also deaths among government troops, but had no immediate toll.



"The attack has ended and the road remains completely closed," the group's chief, Rami Abdel Rahman, said.

The Castello Road was effectively cut-off by government forces on Thursday when they seized a hilltop within range of the key supply line. By Saturday, regime soldiers were within 500m of the road and firing on any vehicle trying to use it.



The observatory said that at least one car was targeted on the road early on Sunday, adding that it was unclear if it carried civilians or fighters.



The regime also shelled the road later on Sunday, it said. On Friday, a man and his two sons were killed by regime forces on the road, the monitor said.

A correspondent for the AFP news agency in eastern Aleppo said opposition factions were preventing civilians from using the route. Ongoing government artillery fire and barrel bomb attacks were reported in the east of the city early on Sunday.

READ MORE: Syria: Army cuts strategic Aleppo road despite truce

About 200,000 people remain in the eastern part of Aleppo, which has been divided between government and rebel control since shortly after fighting in the city erupted in mid-2012.



Syrian President Bashar al-Assad's forces have been trying to cut the Castello Road for more than two years in an attempt to pressure rebel forces in the city.



Their advance on Thursday left residents of the east cut off, with supplies of basic items including food and fuel starting to run low and fears of a lengthy government siege.



Syria's government has been accused of using sieges to pressure rebel forces, and the UN says nearly 600,000 Syrians live in besieged areas, most surrounded by government forces although rebels also use the tactic.



Following the government advance, rebel forces on Friday retaliated with waves of rocket fire into the regime-held west of Aleppo, killing 45 people, according to Syria's state news agency SANA.