BEIRUT (AFP) - Syrian Islamist rebels bombarded a campaign rally in support of President Bashar al-Assad's re-election in a controversial June 3 poll, killing at least 21 people, a monitoring group said.

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

Assad faces two little known challengers in next month's vote and is widely expected to clinch a third seven-year term in office despite the raging civil war.

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

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

It had initially given a death toll of 22.

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."

The June 3 poll will be held only in regime-controlled areas and has been branded as a farce by the exiled opposition and its Western supporters.