Al-Qaeda's branch in Iraq likely carried out recent suicide bombings in Syria and has infiltrated opposition forces fighting President Bashar al-Assad's regime, the US spy chief said Thursday.

Bombing attacks in Damascus and Aleppo since December "had all the earmarks of an Al-Qaeda-like attack," James Clapper told the Senate Armed Services Committee.

"And so we believe Al-Qaeda in Iraq is extending its reach into Syria," he said.

His comments confirmed earlier reports that US officials suspected Al-Qaeda's hand in the bombings and follows a recent video message from Al-Qaeda leader Ayman al-Zawahiri in which he endorses the uprising against Assad's rule.

Iraq's deputy interior minister told AFP this month that Al-Qaeda was moving guns and militants from Iraq into Syria.

Clapper voiced concern that Al-Qaeda militants had inserted themselves inside a divided opposition amid the spiraling violence that activists say has left more than 6,000 people dead since March 2011.

"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," said Clapper, director of national intelligence.

Defense Secretary Leon Panetta, speaking at a news conference with his German counterpart, Thomas De Maiziere, echoed Clapper's view but said Al-Qaeda's influence on the opposition remained unclear.

Asked if Washington could support opposition forces given Al-Qaeda's presence, Panetta said: "I think just the fact that they're present concerns us. As to what their role is, and how extensive their role is, I think that remains to be seen."

Clapper said the opposition was deeply fragmented and that the Assad regime appeared able to hold on to power for the moment as it pressed ahead with the violent repression of protests.

He added there was no sign the stalemate would end anytime soon.

The US spy chief has previously told lawmakers that it was only a matter of time before the regime fell, but predicted a protracted struggle.

Clapper also said US intelligence agencies were monitoring Syria's "extensive network" of chemical weapon stockpiles, which he said presented a bigger challenge than Libya's arsenal did before the fall of dictator Moamer Kadhafi.

"Although to this point, and we're watching these very carefully, they appear to be secure," he said.

Clapper said he expected Al-Qaeda's role, rifts within rebel ranks and the presence of chemical weapons would "affect any discussion (about) coming to some assistance" in support of the opposition.