Beirut (AFP) - Al-Qaeda's affiliate in Syria battled Western-backed rebels Friday as the jihadists pressed their bid to seize control of northern areas, a monitoring group and rebels said.

The fighting comes nearly three months after Al-Nusra Front expelled another group of Western-backed opposition fighters from Idlib province.

The Syrian Observatory for Human Rights said the jihadists launched their offensive against the Western-armed Hazem movement on Thursday in Aleppo province.

"The jihadists expelled the rebels from Regiment 111, once a regime army base that Hazem had taken over," Observatory director Rami Abdel Rahman told AFP.

"Then on Friday, the fighting spread to the north of Idlib province."

The Hazem group confirmed the attack.

"The so-called Al-Nusra Front attacked the movement's positions and checkpoints in the west of Aleppo province ... We in the Hazem Movement will defend ourselves until the last drop of blood," it said in a statement.

Hazem is mainly present in northern Syria. Last year, it was the first to receive US-made anti-tank missiles from its Western backers.

In November, Al-Nusra jihadists expelled the Western-backed Syrian Revolutionaries Front from Idlib.

Elsewhere in Idlib, Islamist rebels detonated explosives planted in a tunnel under an army position in the Jabal al-Arbaeen area, killing at least one soldier and wounding around 24 others, according to the Observatory.

Rebels from Suqur al-Sham group, a component of the massive rebel Islamic Front alliance, distributed videos claiming responsibility for the attack.

One video showed a huge cloud of smoke rising above the site of the explosion.

State news agency SANA, for its part, said the army repelled an attack on Jabal al-Arbaeen, "killing a large number" of rebels.

Syria's conflict began in March 2011 as a popular revolt seeking democratic change, but later morphed into civil war after President Bashar al-Assad's regime launched a brutal crackdown against dissent.

More than 200,000 people have since been killed in a devastating conflict that has driven half of Syria's population from their homes.