A Syrian government official says a car bomb has exploded in a predominantly Alawite district of the central city of Homs, killing 36 people.

The official says Tuesday's bombing also wounded more than 85 people, most of them civilians.

The official, who is in Homs, spoke to The Associated Press over the telephone on condition of anonymity because he was not authorized to talk to media.

Syria state TV only said that a car bomb exploded near the Zahra district of Homs, causing "a large number" of casualties. The Britain-based Syrian Observatory for Human Rights says at least 13 people, including five children, were killed and that more than 40 were wounded in the bombing.

Different casualty tolls are common in the immediate aftermath of large bombings.

Mortar shells hit Damascus

Meanwhile, four mortar shells exploded on Tuesday in central Damascus, killing 14 people and wounding scores, state media said.

The attacks in the Syrian capital come a day after President Bashar al-Assad announced his candidacy for the June 3 presidential elections, a race he is likely to win amid a raging civil war that initially started as an uprising against his rule.

The official SANA news agency said four shells struck in the capital's Shaghour neighbourhood on Tuesday morning. State TV said 14 people were killed in the attacks and 86 were injured.

An official at the Damascus Police Command told The Associated Press that two of the mortar shells hit a school complex. The official spoke on condition of anonymity in line with government regulations.

No one immediately claimed responsibility for the attack. Rebels have frequently fired mortars into the capital from opposition-held suburbs.

SANA blamed the attacks on terrorists — a term used by Assad's government for rebels fighting to oust him. Many of the opposition-held neighbourhoods around Damascus have been under a crippling government blockade for months with no food and medicine allowed to reach trapped civilians inside.

Tuesday's attacks occurred just hours after an international rights organization accused Assad's forces of indiscriminately targeting civilians and civilian infrastructure with crude bombs in rebel-held districts of the northern city of Aleppo.

Barrel bombs

Human Rights Watch said its staff has documented 85 locations in Aleppo's opposition-held districts that government aircraft shelled with barrel bombs — makeshift, shrapnel-packed explosive devices rolled out of helicopters.

The New York-based group identified the locations after interviewing witnesses and analyzing satellite imagery and video and photographic evidence, the report said.

Mideast Syria

Syrians carry their belongings from a building that was hit by the airstrike. (Aleppo Media Centre AMC/AP)

The attacks in Aleppo occurred between Feb. 22 and April 2. The locations, identified by HRW, sustained damage that is "consistent with the detonation of barrel bombs," the report said. The organization also has evidence that government forces fired hundreds of mortar and heavy artillery shells during those 40 days.

Aleppo, Syria's largest city, has been carved up into rebel- and government-held neighbourhoods since the opposition launched an offensive in the north in mid-2012.

HRW also criticized the opposition for firing mortars into populated, residential areas, but the rebel groups said government forces are behind most of the attacks that target residential areas and civilians.

The UN Security Council is meeting on Wednesday to review whether Syria's warring sides are complying with a resolution demanding the cessation of the use of barrel bomb and other weapons in populated areas.

Syria's conflict started as largely peaceful protests against Assad's rule in March 2011. It turned into a civil war after some opposition supporters took up arms to fight a brutal government crackdown on dissent. More than 150,000 have been killed, activists say, and millions have been displaced by the fighting.