(CNN) -- Armed rebels fighting the regime of Syria's Bashar al-Assad have committed "serious human rights abuses," an influential human rights watchdog said Tuesday.
In an open letter to leaders of the opposition, Human Rights Watch cites "increasing evidence" of kidnappings, torture and executions and calls on those forces "to refrain from engaging in these unlawful practices."
The report emerged as the death toll continued to mount in Syria, with dozens of deaths recorded Tuesday by activists. U.N. Security Council members and other world powers worked to address the crisis, which started a year ago when the government began a violent crackdown against protesters.
"We have no time to waste, no time to lose," U.N. Secretary-General Ban Ki-moon told reporters in Indonesia. "The situation has reached unacceptable, intolerable situation now. ... I sincerely hope that the international community will continually speak in one voice and particularly the Security Council, I hope, will be able to be united so they can speak in one voice."
Al-Assad's regime has been denounced across the globe for its apparent slaughter of civilians in an attempt to quash the opposition over the last year.
The United Nations, Syrian activists and groups such as Human Rights Watch have documented widespread violations by Syrian government forces, including the widespread use of torture, arbitrary detentions and indiscriminate shelling of neighborhoods. But abuses by anti-government forces also have been documented during the crisis.
"The Syrian government's brutal tactics cannot justify abuses by armed opposition groups," said Sarah Leah Whitson, Middle East director at Human Rights Watch. "Opposition leaders should make it clear to their followers that they must not torture, kidnap or execute under any circumstances."
Human Rights Watch says the protest movement had been "overwhelmingly peaceful" until September, when reports emerged of military defectors and residents taking up arms to defend themselves against government raids and to strike checkpoints and security sites.
"The intensity of the fighting has increased since early February 2012, when the government began large-scale military attacks against opposition strongholds throughout the country," the group said.
The past year has seen the formation of a resistance group of anti-government military defectors called the Free Syrian Army and a political opposition movement called the Syrian National Council.
Human Rights Watch said many of the anti-government groups reported to be carrying out abuses do not appear to belong to an organized command structure or to be following Syrian National Council orders.
"But Syria's opposition leadership has a responsibility to speak out and condemn such abuses," Human Rights Watch said. "On March 1 the SNC created a military bureau to liaise with, unify, and supervise armed opposition groups, including the Free Syrian Army," the group said.
Human Rights Watch said those kidnapped include security forces, government-supported militia members known as shabiha and their backers. Security force members and civilians also have been tortured and executed, it said.
"Certain armed attacks by opposition groups were motivated by anti-Shia or anti-Alawite sentiments arising from the association of these communities with government policies," Human Rights Watch said. Al-Assad's government is dominated by the minority Alawite community, whose faith is an offshoot of Shiite Islam. About three-quarters of the Syrian population are Sunnis.
Syrian National Council member Sheikh Anas Airout notes "some exceptional situations and unfortunately these are reactions to the horrors, the crimes and the atrocities that the regime keeps committing against our people." But he told CNN that the abuses are "totally unacceptable."
"For a year now, the Syrian opposition didn't resort to any unacceptable act against any pro-Assad civilian or even the soldiers who are killing our people. We encourage our free men to show mercy to our captives because we want to prove to the world that we are better than the Assad regime and we will always be. We do not want to repeat the regime's same mistakes. Saying that, we have to keep it in mind that when we see the killing machine of Assad and his thugs slaughtering our people every hour of the day and the whole world is sitting aside and watching, we know and we understand that there would be some elements who would commit such acts."
Free Syrian Army Lt. Riad Ahmed said a "few incidents are a drop in a bucket in comparison to what the regime is committing."
"The FSA have prisoners and we treat them based on the Geneva Convention. We know that our soldiers and our civilians, men, women and children are being tortured, beaten up and even killed by the Assad thugs," Ahmed said.
The opposition Local Coordination Committees of Syria said at least 52 people were killed across Syria Tuesday.
The Syrian regime launched new assaults on Homs province Tuesday, killing one of the first organizers in the Syrian uprising, an opposition group said.
Heavy shelling in the city of Homs claimed the life of 23-year-old Abdul Rahman Orfalli, the Homs Coordination Committee said. The group said Orfalli helped organize the first protests in the city last March.
He had been arrested twice and tortured during a five-month detainment before returning to Homs to lead demonstrations, the group said.
The LCC reported the discovery in Idlib province of three Free Syrian Army members who had been executed, each shot in the head. The group said the men also "had insults written on their bodies."
It also said the army stormed a field hospital in Idlib and killed three members of the medical staff.
U.N. Security Council members were expected to discuss Syria in closed session on Tuesday. They could support a "presidential statement" supporting the mission of Kofi Annan, the special joint U.N.-Arab League envoy to Syria. U.N. presidential statements aren't legally binding, like resolutions, and require unanimous support.
A five-member U.N. team with expertise in politics, peacekeeping and mediation was in Syria, according to Eduardo del Buey, deputy spokesman for the U.N. secretary-general. The team will stay as long as it is making progress toward goals set by Annan, del Buey said.
The U.N. effort is viewed as an attempt to lure China and Russia -- two countries that have refused to formally condemn the Syrian regime -- to join others in pressuring Syria to cooperate with Annan, who met with al-Assad this month and laid out a series of proposals intended to end the crisis.
Russia and China have said they want an end to the violence but would not place the blame squarely on the regime. Both countries also have major trade ties with Syria.
Ban urged an end to violence by all sides, dialogue for a political solution and unhindered access for humanitarian aid workers.
U.N. officials say the Syrian crisis has killed more than 8,000 people, while opposition activists put the toll at more than 9,000 -- most of them civilians.
The Syrian regime consistently blames "armed terrorist groups" for the violence. CNN cannot independently confirm reports of casualties or attacks in Syria because the government has severely restricted the access of international journalists.
CNN's Saad Abedine, Amir Ahmed and Mick Krever contributed to this report.