Members of Syria's armed opposition have been accused of committing serious human rights abuses during the yearlong uprising against the government of President Bashar al-Assad.

Human Rights Watch said Tuesday that since September, some Syrian opposition fighters have been engaged in the kidnapping and torture of government soldiers and members of pro-Assad militia groups.

The report comes as violence again intensified in Syria. Government forces have continued shelling neighborhoods in Homs --  15 people were reportedly killed by Assad's troops in Homs, Hama and Damascus on Tuesday.

“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.”

The rights organization added that most of those committing abuses are not associated with the Syrian National Council -- the internationally recognized opposition group leading the anti-Assad efforts. The Free Syrian Army, the SNC's armed wing, was implicated in the report but not connected to any particular incident.

However, HRW insisted that it is up to the SNC to condemn the abuses and to prevent them from continuing.

“It is imperative for armed elements of the Syrian opposition to protect human rights,” Whitson said. “They need to make it clear that they envision a Syria that turns the page on Assad-era violations and welcomes all – regardless of their religious group or background – without discrimination.”

Sami Ibrahim, of the Syrian Network for Human Rights, responded to the HRW report, telling MSNBC that the rape and torture being committed by Assad's army is changing peaceful civilians into a terrorist people.”

“What he [Assad] is doing is [forcing] a generation to go to the dark way, to go to the darkness. The Assad regime, this criminal dictatorship, transfers the people from normal people, peaceful people, to another side we cannot control,” he said.

Eyewitnesses told HRW that armed gangs are associating themselves with the opposition as a way to justify kidnapping civilians and holding them for ransom.

On Tuesday, United Nations Secretary-General Ban Ki-moon again condemned the violence in Syria, saying there is no time to waste, no time to lose in finding a peaceful resolution.

The situation has reached unacceptable, intolerable situation now, the U.N. chief said from Indonesia.

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.

Russia and China have blocked strong action by the council.