Backers of President Bashar Assad hold his portraits and wave Syrian flags during a demonstration in support of his candidacy for presidential elections, in Damascus. Picture: SANA

Damascus - Syrian rebels have shelled a rally in support of President Bashar al-Assad's re-election, killing 21 people, after the army cut a key rebel supply route to the main northern city of Aleppo.

Mortar fire, which hit a tent where Assad supporters had gathered in the southern city of Daraa late on Thursday, also wounded at least 30 people, the Syrian Observatory for Human Rights said.

Assad faces two little-known challengers in the June 3 vote that exiled rebels and their Western backers have labelled a farce, and is widely expected to clinch a third seven-year term.

The Observatory said a child was among the dead in the attack, the first on Assad supporters since campaigning got underway earlier this month.

Pro-government militiamen were also among those killed in the attack, carried out by an Islamist rebel brigade, the Britain-based watchdog said.

Observatory director Rami Abdel Rahman told AFP the attack “is a clear message from rebels to the regime that there is not one safe area in which to hold the election.”

As it is, the vote will only take place in regime-controlled areas.

Earlier Thursday, the army broke a 13-month rebel siege on Aleppo prison.

That put regime forces within reach of Castello Road, which rebels in Aleppo have relied on as a supply route leading to a rear base in the countryside.

Rebels had repeatedly attacked the prison since April 2013, hoping to free approximately 3 500 detainees reportedly being held in dire conditions.

The prison had nearly 4 000 inmates before the siege, including Islamists, but the Observatory said about 600 prisoners had died due to poor humanitarian conditions and bombardment.

In that vein, UN Secretary-General Ban Ki-moon said humanitarian access throughout Syria was worsening, blaming the government in particular but also the rebels.

His report came after Damascus allies China and Russia vetoed a draft UN Security Council resolution that would have referred Syria to the International Criminal Court to examine chemical attacks, systematic torture, barrel bombings and blocked aid.

Ban said Damascus was still stopping convoys from entering Syria from Iraq, Jordan and Turkey in violation of a Security Council resolution adopted in February.

The report said more than 3.5 million Syrians continue to have “unpredictable and woefully inadequate” access to humanitarian assistance.

The conflict flared after a bloody crackdown on a popular uprising in March 2011. It is estimated to have claimed at least 162 000 lives and displaced nearly half the population.

“Parties to the conflict, particularly the government of Syria, continue to deny access for humanitarian assistance in a completely arbitrary and unjustifiable manner,” it said.

“I remind the Council that intentionally using starvation as a weapon of war is a gross violation of international humanitarian law,” the report said.

Ban called on the council to “urgently consider what steps it will now take to secure compliance with its demands,” including lifting sieges and opening cross-border points.

The council, which has so far failed to reach agreement, is due to debate the report on May 29.

Thursday's action by China and Russia at the Security Council was the fourth time they have blocked Western resolutions relating to the conflict.

The other 13 council members all voted in favour of the draft, which would have seen crimes committed by all sides in the country's war examined.

Meanwhile, the world's chemical watchdog said the last of Syria's chemicals agents were packed and ready to be taken to Latakia port and out of the country when the security situation permits.

Syria has already shipped out 92 percent of its stockpile from the Mediterranean port of Latakia. - AFP