Rebels in Syria have seized control of sections of the country's international borders and torched the main police headquarters in Damascus.

Officials in neighbouring Iraq confirmed that rebels were now in control of the Syrian side of the main Abu Kamal border checkpoint on the Euphrates River highway, one of the major trade routes across the Middle East.

Syrian rebels also claimed control of at least two border crossings into Turkey at Bab al-Hawa and Jarablus, in what appeared to have been a coordinated campaign to seize Syria's frontiers.

It is a dramatic advancement after the assassination of President Bashar al-Assad's closest lieutenants on Wednesday - to which government forces responded by sending helicopter gunships and artillery into the capital.

The next few days will be critical in determining whether Assad's government can recover from the devastating blow of Wednesday's bombing, which killed his brother-in-law, his defence minister and a top general.

After the attacks Assad failed to appear in public for over 24 hours, adding to the sentiment that his power is quickly evaporating.

In Damascus, activists said rebels were now in control of the capital's northern Barzeh district, where troops and armoured vehicles had pulled out.

The army has also pulled out of the towns of Tel and Dumair north of Damascus after taking heavy losses, they said.

However these reports could not be confirmed as the Syrian government has restricted access by international journalists.