Beirut (AFP) - Syrian armed opposition groups, among them jihadists but also Western-backed rebels, have indiscriminately targeted civilians in violation of the laws of war, Human Rights Watch said on Monday.

In a new report, the New York-based group said it had documented dozens of opposition attacks against civilians in government-held parts of Syria.

The group's deputy Middle East and North Africa director Nadim Houry said there had been a "race to the bottom in Syria, with rebel groups mimicking the ruthlessness of government forces."

"Civilians are paying the price, be it in government or rebel-held areas, with an inadequate international response," Houry said.

The group documented car bombings and indiscriminate shelling in government-held areas between January 2012 and April 2014, focusing mostly on areas investigators were able to visit.

The report said several dozen car bomb attacks in Damascus and Homs provinces had hit areas with no government military targets, and often targeted districts occupied by religious minorities.

"Besides being indiscriminate, many of these attacks seemed primarily intended to spread terror among the civilian population," the report said.

Among the worst of the bombings was an attack in October last year outside a school in the Akrameh neighbourhood of central Homs city that killed more than 45 children.

HRW also documented regular indiscriminate shelling by opposition groups on the capital Damascus and Homs city, mostly against civilian areas, and in some cases schools full of children.

The groups responsible for the indiscriminate attacks ranged from jihadists like the Islamic State group to rebels grouped under the Western-backed Free Syrian Army, the report said.

At least 10 of the bomb attacks it investigated were claimed by jihadists from the Islamic State or Al-Qaeda's Syrian affiliate Al-Nusra Front, but FSA groups also regularly claimed responsiblity for shelling civilian areas in Damascus, HRW said.

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The group said rebel assertions that targeting government-held areas was legitimate retaliation "carry no validity under the laws of war."

"All parties to the conflict should end all deliberate, indiscriminate, and disproportionate attacks against civilians," the group added.

It urged international figures with influence to "condemn all sides for unlawful attacks," and warned that governments providing military aid to combatants violating the laws of war risked complicity in those violations.

More than 215,000 people have been killed in Syria since an anti-government movement that began in March 2011 descended into a violent civil war.

Syria's government has also been accused of widespread abuses, including indiscriminate aerial bombardment, endemic torture and the use of chemical weapons.