US campaign group Human Rights Watch (HRW) says elements of Syria's armed opposition have carried out serious human rights abuses, including kidnapping, torture and execution.
The group says the "brutal tactics" of the government cannot justify abuses by armed opposition groups.
HRW calls on the opposition leadership to speak out and condemn those abuses.
It made the statement in an open letter to the main opposition coalition, the Syrian National Council (SNC).
According to HRW, abuses include kidnapping for ransom, detention, and torture of security force members, government supporters, and people identified as members of pro-government militias, called Shabiha.
HRW says it has also received reports of executions by armed opposition groups of security force members and civilians.
"The Syrian government's brutal tactics cannot justify abuses by armed opposition groups," said Sarah Leah Whitson, Middle East director at HRW.
"Opposition leaders should make it clear to their followers that they must not torture, kidnap, or execute under any circumstances."
The group said many of the anti-government groups reported to be carrying out abuses did not appear to belong to an organised command structure or to be following Syrian National Council orders.
HRW has frequently accused Syria's government of abuse over the past year of conflict.
The UN says more than 8,000 people have been killed in the year-long uprising, while tens of thousands of people have fled their homes.
The Syrian government has been trying to suppress an uprising inspired by events in Tunisia, Egypt and Libya. The UN says thousands have been killed in the crackdown, and that many more have been detained and displaced. The Syrian government says hundreds of security forces personnel have also died combating "armed terrorist gangs".
The family of President Bashar al-Assad has been in power since his father, Hafez, took over in a coup in 1970. The country underwent some liberalisation after Bashar became president in 2000, but the pace of change soon slowed, if not reversed. Critics are imprisoned, domestic media are tightly controlled, and economic policies often benefit the elite. The country's human rights record is among the worst in the world.
Syria is a country of 21 million people with a Sunni Muslim majority (74%) and significant minorities of Alawites - the Shia heterodox sect to which Mr Assad belongs - and Christians. Mr Assad promotes a secular identity for the country, but he has concentrated power in the hands of family and other Alawites. Protests have generally been biggest in Sunni-dominated areas.
Under the sanctions imposed by the Arab League, US and EU, Syria's two most vital sectors, tourism and oil, have ground to a halt in recent months. The IMF says Syria's economy contracted by 2% in 2011, while the value of the Syrian pound has crashed. Unemployment is high, electricity cuts trouble Damascus, and critical products like heating oil and staples like milk powder are becoming scarce.
Pro-democracy protests erupted in March 2011 after the arrest and torture of a group of teenagers who had painted revolutionary slogans on walls at their school in the southern city of Deraa. Security forces opened fire during a march against the arrests, killing four. The next day, the authorities shot at mourners at the victims' funerals, killing another person. People began demanding the overthrow of Mr Assad.
The government has tried to deal with the situation with a combination of minor concessions and force. President Assad ended the 48-year-long state of emergency and introduced a new constitution offering multi-party elections. But at the same time, the authorities have continued to use violence against unarmed protesters, and some cities, like Homs, have suffered weeks of intense bombardment.
The opposition is deeply divided. Several groups formed a coalition, the Syrian National Council (SNC), but it is dominated by the Sunni community and exiled dissidents. The SNC disagrees with the National Co-ordination Committee (NCC) on the questions of talks with the government and foreign intervention, and has found it difficult to work with the Free Syrian Army - army defectors seeking to topple Mr Assad by force.
International pressure on the Syrian government has been intensifying. It has been suspended from the Arab League, while the EU and the US have imposed sanctions. However, there has been no agreement on a UN Security Council resolution calling for an end to violence. Although military intervention has been ruled out by Western nations, there are increasing calls to arm the opposition.
Correspondents say a peaceful solution seems unlikely. Syria's leadership seems intent on crushing resistance and most of the opposition will only accept an end to the regime. Some believe the expected collapse of Syria's currency and an inability to pay salaries may be the leadership's downfall. There are fears, though, that the resulting chaos would be long-lasting and create a wider conflict.
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