Insurgent groups in Syria have carried out scores of indiscriminate attacks that have killed and maimed civilians in violation of the laws of war, a Human Rights Watch report said on Monday.

The report said armed groups could not point to abuses by government forces and allied militias to justify their own violence, which it said had often targeted areas with a high concentration of religious minorities.

"We've seen a race to the bottom in Syria, with rebel groups mimicking the ruthlessness of government forces with devastating consequences for civilians," saidNadim Houry, HRW's deputy Middle East director.

The Syrian crisis started in March 2011 with Arab Spring-inspired protests against President Bashar al-Assad. The uprising turned into armed conflict as the security forces cracked down on protesters.

Four years on, more than 200,000 people have been killed in a civil war pitting the army and allied militias against a range of insurgent groups, including hardline jihadists such as Islamic State and mainstream rebels.

The report covered attacks between January 2012 to April 2014 in and around Damascus and Homs. Some attacks were claimed by groups such as al Qaeda's Syria wing Nusra Front and the ultra-hardline Islamic State, HRW said.

However, members of the "Free Syrian Army" and other rebel groups also appear to have carried out deliberate, deadly attacks on civilian areas, the HRW report found.

Free Syrian Army is a name adopted by a plethora of mainstream rebel groups that often operate independently of each other. Some of these groups have received support from Assad's Western and Arab foes.

The research was based on victim and witness accounts, on-site investigations, videos and information on social media. It described attacks using car bombs, mortars and rockets.

The report documented 17 car bombings and other explosions in the Damascus countryside, central Damascus and various locations in Homs.

Many of the areas targeted have a large population of religious minorities including Christians, Alawites, Shi'ites and Druze which are seen by Sunni Muslim insurgents as supportive of the government, the report said.

Assad is an Alawite and his allies include the Shi'ite Islamist government in Iran.

The report urged the United Nations Security Council to refer the situation in Syria to the International Criminal Court and impose an arms embargo on forces implicated in widespread or systematic abuses, whichever side they were fighting on.