DAMASCUS - Washington put a key Syrian rebel group on its terror blacklist on Tuesday citing Al-Qaeda links, a day after the jihadist faction showed its power in the battlefield by capturing a key army base.

The US move came amid growing Western concern that Al-Qaeda loyalists have been hijacking the 21-month revolt against President Bashar al-Assad's rule and could turn any weaponry supplied to the rebels against Western targets.

Washington balanced its move with the announcement of fresh sanctions against pro-Assad militias.

But the blacklist of the Al-Nusra Front marked a major shift in US policy towards the rebels which had previously been tolerant of the large Islamist element within their ranks.

The US State Department said that despite its efforts to portray itself as part of the legitimate Syrian opposition, Al-Nusra was a front for the Al-Qaeda in Iraq (AQI) organisation that mounted a deadly insurgency against US troops in Syria's eastern neighbour which peaked in 2006-7.

"It is, in fact, an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes," it said.

The Al-Nusra Front's fighters, many of them jihadist volunteers from around the Islamic world, were instrumental in the fall of the army's massive Sheikh Suleiman base in northern Syria on Monday after a months-long siege.

Its role in the seizure of the garrison, the government's last between second city Aleppo and the Turkish border, undercut the military influence of the mainstream rebel Free Syrian Army (FSA) which the West has been counting on to rein in the jihadists.

An AFP journalist who witnessed the clashes around Sheikh Suleiman said many fighters were from other Arab countries and Central Asia.

The US Treasury Department designated two of the Al-Nusra Front's senior leaders, Maysar Ali Musa Abdallah al-Juburi and Anas Hasan Khattab, for sanctions.

It also imposed sanctions on two armed militias supporting the Assad regime -- Jaysh al-Sha'bi and Shabiha -- as well as two Shabiha commanders.

"These militias have been instrumental in the Assad regime's campaign of terror and violence against the citizens of Syria," the Treasury Department said.

The United States "will target the pro-Assad militias just as we will the terrorists who falsely cloak themselves in the flag of the legitimate opposition," said David Cohen, under secretary for terrorism and financial intelligence.

At the same time Washington said that it had reason to ease the urgent concerns it had expressed in recent weeks about the dangers of Damascus resorting to use of its chemical weapons stockpiles against the rebels.

US Defence Secretary Leon Panetta said Syria had not taken any new steps in recent days that signal a readiness to use its arsenal.

"At this point the intelligence has really kind of levelled off. We haven't seen anything new indicating any aggressive steps to move forward in that way," Panetta told reporters aboard his plane before landing in Kuwait.

"But we continue to monitor it very closely and we continue to make clear to them that they should not under any means make use of these chemical weapons against their own population."

Inside Syria, Islamist rebels loyal to Muslim Brotherhood-backed Liwa al-Tawhid assaulted a military school in battlefield Aleppo province, the Syrian Observatory for Human Rights said.

"The school is very important for its size and location," Observatory director Rami Abdel Rahman said, adding that it schooled some 3,000 cadets.

In footage posted on the Internet with the group's logo, five gunmen were seen firing automatic weapons from behind a hill toward a building in a wooded area, as explosions are heard in the background. The video's authenticity could not be verified.

With the death toll now topping 42,000, according to the Observatory's figures, the UN High Commissioner for Refugees said the number of Syrian refugees in neighbouring countries and the wider Arab world had now passed half a million.

"And these numbers are currently climbing by more than 3,000 a day," UNHCR spokeswoman Melissa Fleming told reporters in Geneva.