Syrian rebels have captured a major army post in the southern city of Deraa after nearly two weeks of intense fighting, activists said.

The Britain-based Syrian Observatory for Human Rights, which has a network of activists around the country, said rebel forces led by members of the the Nusra Front, captured the checkpoint on Friday after a two-week siege.

Rami Abdulrahman, the head of the Observatory, said the fall of the army post was strategically significant for Deraa, where protesters first marched against four decades of Assad family rule from the city's Omari mosque in March 2011.

"Now the army is under threat there. The rebels haven't liberated all of the old city. There are still two neighbourhoods with soldiers, but this could change the balance of power there," Abdulrahman told Reuters.

The Syrian conflict began in Deraa as a peaceful protest movement against President Bashar al-Assad's rule, but has spread across the country and degenerated into civil war.

He added that the capture opens the way for rebels to take the southern neighbourhood of Manshiyeh that is close to the Jordanian border.

Earlier, the Observatory said intense shelling by Syrian government troops on the village of Karak in Daraa province killed at least 10 women and girls overnight.

The United Nations has estimated that more than 6,000 children are among some 93,000 people killed in Syria's more than two-year-old conflict, which started with largely peaceful protests against the rule of Assad.

The United States and its allies recently said they will help arm the rebels amid reports that Washington's Gulf allies have already sent much-coveted anti-tank missiles to selected groups of fighters.

The US is still trying to sort out which rebels exactly will be given weapons and how, fearing that advanced arms may fall in the hands of extremists in the rebel ranks.