|Al Jazeera's James Bays reports on claims that al-Qaeda fighters have entered Syria from Lebanon
The US intelligence chief says al-Qaeda's branch in Iraq may have carried out recent bombings in Syria and infiltrated armed opposition groups fighting President Bashar al-Assad's forces.
Bombings in Damascus and Aleppo since December targeting intelligence buildings "had all the earmarks
of an al-Qaeda-like attack", James Clapper told the Senate Armed Services Committee on Thursday.
"And so we believe al-Qaeda in Iraq is extending its reach into Syria."
Syrian state media said "suicide bombings" on December 23 in Damascus killed dozens of people and the government blamed al-Qaeda for the attacks. The opposition claimed that authorities had carried out the attacks.
Bombings in Aleppo last Friday also killed dozens, according to state media. Arif al-Hummoud, a commander of the Free Syrian Army, a name used by various armed groups, said opposition fighters had carried out an attack on security targets in Aleppo but were not responsible for the blasts.
Clapper voiced concern that al-Qaeda fighters had inserted themselves inside armed opposition forces amid the spiralling violence that activists say has left more than 7,000 people dead since March last year.
"Another disturbing phenomenon that we've seen recently, apparently, is the presence of extremists who have infiltrated the opposition groups ... The opposition groups in many cases may not be aware that they're there."
"We've seen evidence of Sunni extremists," he said. "Can't label them specifically as al-Qaeda, but similar ilk who are infiltrating the oppositionist groups."
In a video message released on Saturday, al-Qaeda chief Ayman al-Zawahri called on Muslims to support Syrian rebels.
"Wounded Syria is still bleeding day after day, and the butcher [Assad] isn't deterred and doesn't stop,” said al-Zawahri, who took over al-Qaeda after Osama bin Laden was killed by US special forces last May.
"However, the resistance of our people in Syria is escalating and growing despite all the pains, sacrifices and blood."
In-depth coverage of escalating violence across Syria
At the Pentagon, Leon Panetta, the US defence secretary said the crisis in Syria had become "much more serious" and worrisome to the US as a result of indications that al-Qaeda had infiltrated the opposition.
"It does raise concerns for us that al-Qaeda is trying to assert a presence there," he said. "As to just what their role is and how extensive their role is, I think that still remains to be seen."
Al Jazeera's Anita McNaught, reporting from neighbouring Turkey, said al-Qaeda knew Syria extremely well.
"No one should be surprised if al-Qaeda shows a presence in Syria. Syria was the route by which al-Qaeda entered Iraq. We know from captured fighters in Iraq, from computers and documents captured with them, that there was a whole infrastructure in Syria that dedicated itself to this safe passage of fighters through Syrian territory."
Iraq's deputy interior minister told the AFP news agency earlier this month that al-Qaeda was moving guns and fighters from Iraq into Syria.
"We have intelligence information that a number of Iraqi jihadists went to Syria," Adnan al-Assadi said, adding that "weapons smuggling is still ongoing" from Iraq into Syria.
He said that the price of a Kalashnikov assault rifle has risen from between $100 and $200 to between $1,000 and $1,500.
"The weapons are being smuggled from Mosul through the Rabia crossing to Syria, as members of the same families live on both sides of the border," he said. And "there is some smuggling through a crossing near Abu Kamal," Assadi said, referring to a Syrian town.
Syrian troops have beefed up their presence on the border with Lebanon in an attempt to stop smuggling of weapons through the porous border, where smuggling business has been flourishing for decades.
Fayez Ghosn, the Lebanese defence minister, said in December that al-Qaeda fighters were operating on the border, disguised as Syrian opposition activists.
But a week later, Najib Mikati, Lebanon's prime minister, counter that statement, saying there was no evidence that al-Qaeda was operating in the country.
"There is no firm evidence of an al-Qaeda presence in [border area] Ersal," Mikati told journalists late on Wednesday, according to Lebanon's state National News Agency.
"The military entered Ersal after being informed there was a person in the area who was perhaps tied to an international terrorist group but there was no information about the presence of any organised groups or any particular organisation."
Omar Bakri, a hardline cleric who the Syrian government has pointed out as the head of al-Qaeda in Lebanon, told Al Jazeera that he did not think the group was active in Syria.