Extremist rebel groups in Syria have carried out death sentences on prisoners by cutting their throats, an opposition human rights group reported Tuesday.

Two videos released by the Britain-based Syrian Observatory for Human Rights appeared to show rebels cutting the throats of a prisoner accused of collaborating with the regime and a captured soldier.

The video of the first apparent killing begins with a man, holding a long thin knife, announcing a verdict against a prisoner for collaborating with the regime and "spreading corruption on earth".

He is then shown handing the knife to another man who cuts the throat of a bound and blindfolded man lying on the ground.

Rami Abdel-Rahman, head of the Observatory, said that one of the two killings took place in Aleppo province, northern Syria, in January while the other was carried out in Daraa on an unknown date.

"This is an unacceptable act towards Syrian citizens," Abdel-Rahman told DPA, arguing that the uprising against Syrian President Bashar Assad "started with a call for democracy and justice. Such acts are simply against what we believe in."

The videos could not be independently verified. Abdel-Rahman declined to name the groups responsible for the killings.

Western countries have increasingly expressed fears lately that extremist Islamist groups are gaining influence among Syrian rebels.

In December, the United States listed one of the most prominent Islamist rebel groups, the Al-Nousra Front, as a terrorist organization.

State media reported that scores of "terrorists" - a term used by the regime to refer to rebels - were killed or injured as the army targeted their positions in the Sheikh Miskeen district, on the eastern outskirts of Daraa.

Nearly two years into the conflict in Syria, the death toll stands at 60,000, according to UN estimates, with some 600,000 refugees and two million internally displaced.

Meanwhile, the pro-regime newspaper Al-Watan, in a first semi-official response to the offer of talks made last week by Syrian opposition leader Moaz Khatib, said the call was significant but came "two years late".

An editorial in Al-Watan rejected Khatib's comments Monday on Al Jazeera television that "the ball is now in the regime's court."

Rather, it said, "two years have gone by, in which we have lost a lot because of the opposition's stubbornness and their refusal to negotiate."