Al-Qaeda's leader has backed the anti-government uprising in Syria, urging the opposition not to rely on the Arab League or the West for help.
In a video message, Ayman al-Zawahiri said the Syrian rebels had the right to use whatever means they saw fit to get rid of a "cancerous regime".
Egyptian-born Zawahiri, 62, took over as head of al-Qaeda after the death of Osama Bin Laden in May 2011.
The Syrian government blamed al-Qaeda for two blasts in Damascus in December.
That double suicide bombing killed 44 people.
'Muddy the waters'
Zawahiri, who has a $25m (£15m) US bounty on his head, addressed his video message to the "Lions of Syria".
He urged them to depend on their own efforts and sacrifices, and not on what he called the "failed states of the Arab League, the West, or Turkey".
"If we want freedom, we must be liberated from this regime. If we want justice, we must retaliate against this regime," he said in the video.
Arab League ministers are meeting in Cairo on Sunday to discuss the crisis in Syria, following the league's decision to suspend its observer mission in the country last month.
Zawahiri called on militants in Iraq, Jordan, Lebanon and Turkey, to rise up and support what he called "their brothers in Syria".
There have already been reports of Islamic militants crossing into Syria from Iraq.
It is the clearest sign yet of involvement by al-Qaeda in the uprising in Syria, as it takes on increasingly an aspect of armed insurgency as well as popular protest, says the BBC's Jim Muir, in neighbouring Lebanon.
US officials are reported to believe the recent suicide car bombings in Damascus and Aleppo were the work of al-Qaeda.
Opposition activists have blamed the Syrian government itself for the attacks.
If violent Islamic extremists are becoming increasingly involved, it is going to muddy the waters of an already very complex situation, our correspondent says.