BEIRUT (Reuters) - A car bomb in a district south of Damascus killed at least six people and possibly many more on Monday, according to Islamic State militants who claimed responsibility for the attack.

The ultra-hardline Sunni group's attack near a Syrian army checkpoint had killed or injured 48 people, a news agency close to the militants said.

Lebanese group Hezbollah's Al Manar television reported the blast was at an army checkpoint and put the death toll at eight.

The British-based Syrian Observatory for Human Rights also said eight people had been killed and the toll was expected to rise because of the number of people with serious injuries.

State media said the bombing was on the outskirts of Sayeda Zeinab, home to Syria's holiest Shi'ite Muslim shrine. The area has already been hit by two bomb attacks this year.

Multiple explosions in February killed scores of people in one of the bloodiest attacks in that area in Syria's five-year conflict, and an Islamic State suicide attack there less than a month earlier killed 70 people.

Syrian government negotiator Bashar Ja'afari said Monday's blast "that four terrorists carried out" had hit a hospital, killing some patients evacuated last week from two rebel-besieged towns in the northwestern province of Idlib.

Separately, rebel shelling of government-held parts of the northern city of Aleppo killed at least 18 people, the Observatory said.

Syrian aerial bombing has killed dozens of civilians in Aleppo's residential area of Bustan al Qasr and areas under the control of rebels in the last few days, it said.

The government says it is planning a campaign to take areas of Aleppo under rebel control and to cut off their only remaining route into the city.

(Reporting by John Davison, Tom Perry and Suleiman al-Khalidi; Additional reporting by Stephanie Nebehay in Geneva; Editing by Louise Ireland)