BEIRUT: An Australian citizen fighting with Syrian rebels in their battle against President Bashar al-Assad's regime has been killed in the northwest of the country, activists said on Wednesday.

"Abu al-Walid al-Australi was killed December 30 in a rebel assault on the Wadi Deif base," Rami Abdel Rahman, director of the Syrian Observatory for Human Rights, told AFP by telephone.

Rebel fighters launched an assault on the Wadi Deif base, one of the regime's last strongholds in the northwest of Syria, on December 28 in a fresh bid to wrest control of the strategic post.

The jihadist group the Al-Nusra Front led the offensive, according to rebels on the ground.

Insurgents captured the nearby town of Maaret al-Numan, located on the important Damascus-Aleppo highway, in October.

Syria's insurgents comprise army deserters, civilians who have taken up arms and foreign fighters, including several Islamist groups.

The Australian government has condemned Assad's regime for its violence against its own people, expelled Syrian diplomats and demanded the president's departure.

The Syrian conflict, which started as a peaceful uprising against Assad in March 2011 but descended into civil war when it was violently suppressed, has killed more than 46,000 people, many of them civilians, according to the Britain-based Observatory.

The Observatory relies on a network of activists and medics on the ground for its information.