Even as Syria’s security forces have tortured and massacred civilians and anti-government activists, armed members of the Syrian opposition have carried out “serious human rights abuses” against Syrian soldiers and some civilians, the New York-based Human Rights Watch said Tuesday.

The watchdog organization made the allegations of kidnapping, torture, detention and other misconduct in a public letter to the main Syrian opposition group, the Syrian National Council, and other opposition groups. The letter urges anti-government leaders to condemn and forbid the abuses.

Those carrying out the abuses do not appear to be answerable to any of the main opposition structures, Human Rights Watch said. But it is nonetheless important for opposition leaders to “make it clear to their followers that they must not torture, kidnap, or execute under any circumstances,” said Sarah Leah Whitson, the organization’s Middle East director.

“The Syrian government’s brutal tactics cannot justify abuses by armed opposition groups,” Whitson said.

Her group quoted a witness called Mazen as saying that three people who worked with the government had been tortured to death in the northern village of Taftanaz. An opposition activist called Samih told Human Rights Watch that members of the Free Syrian Army were kidnapping Syrian soldiers and exchanging them for ransom in the nearby town of Saraqeb.

In addition, Human Rights Watch said it had viewed 25 videos on YouTube that showed instances in which members of the Syrian security forces had been detained by the opposition and forced to confess to crimes under duress. One video showed the apparent execution of a member of one of the pro-government militias known as shabeeha. The man was hanged on a tree.

Foreign reporters have been mostly barred from entering Syria during the year-long uprising, and it is difficult to independently confirm details of what is happening there. But violent clashes between security forces and the armed opposition appear to be continuing.

A rare gun battle broke out early Monday between rebels and security forces in an upscale neighborhood of central Damascus, taking the violence that has ravaged many other parts of Syria to the heart of the capital and undermining government claims that it is asserting control after a year of unrest.

Residents of the western Mezzeh neighborhood, which is home to foreign embassies, government offices and senior officials, described hearing explosions and gunfire over several hours, starting early Monday morning. Many said this was the fiercest fighting yet in the capital since the uprising against President Bashar al-Assad’s rule began a year ago.

Video footage posted on YouTube showed a burned-out apartment on the top floor of a six-story building, with bullet and shrapnel scars in the stairwell indicating a fierce battle.

The government and the rebels gave conflicting accounts of what had occurred. The official Syrian Arab News Agency said the battles erupted when government forces stormed a “terrorist” hideout in an apartment building in the area. It said two gunmen and a member of the security forces were killed, but it gave no further details.

The Free Syrian Army said, however, that its fighters had launched an attack on the Political Security Directorate, a branch of the security forces, using rocket-propelled grenades and machine guns.

Some details of the assertions by the opposition force appeared improbable. Col. Malik al-Kurdi, a spokesman for the Free Syrian Army interviewed at the military refugee camp in Turkey that serves as a de facto headquarters for the rebel group, said that 87 government soldiers were killed in the assault and that no rebels died.

Rami Abdulrahman of the London-based Syrian Observatory for Human Rights said it did appear, however, that armed rebels had carried out an attack in the symbolically significant area, targeting the home of a major general with a rocket-propelled grenade and then taking refuge in a nearby apartment building, where they held out against the security forces for three hours. He cited witnesses as saying that four rebel fighters and two members of the security forces died.

The attack was significant, Abdulrahman said, because it signaled that the government is unable even to secure one of the capital’s most prosperous and politically sensitive neighborhoods.

“To fight for three hours in Mezzeh is something. They attacked the house of a top official in Damascus, and maybe tomorrow they will attack the house of Bashar al-Assad,” he said. “Even with one RPG, it shows they are there. They are trying to say, ‘We are here, and we can go anywhere.’ ”

The attack was intended to boost flagging morale and to remind the government that the Free Syrian Army still has the capacity to fight, Kurdi said. The opposition has recently suffered setbacks in the city of Homs and the northern province of Idlib, where government forces reclaimed many areas that had fallen under rebel control.

“We have threatened the regime many times before to take the fight to the heart of the capital, and now we have done it,” he said. “And we will continue with our attacks because this regime doesn’t understand any language other than force.”

Kurdi added that the attack signaled the launch of a new Free Syrian Army brigade, the first to be based in the capital, called the Brigade of the Martyrs of the Capital, bringing the total number of brigades to more than 30.

The officers camped out in Turkey have almost no control, however, over the ad hoc rebel units fighting on the ground inside Syria, most of which were formed spontaneously at the community level by defected soldiers and residents frustrated with the levels of violence used by the government to suppress the protest movement.

Amid concerns that civilian suffering is growing as the government presses its offensive against areas held by the rebels in the north and center of the country, Russia agreed Monday to a request from the International Committee of the Red Cross to try to exert pressure on the Assad government to allow aid into stricken areas.

In a statement issued after a meeting with ICRC head Jakob Kellenberger, Russia urged all sides in the conflict to observe daily cease-fires to allow aid into stricken areas.

“The sides called on the Syrian government and all armed groups who oppose it to agree without delay to daily humanitarian pauses,” the Russian Foreign Ministry said.

Many Syrian activists blame Russia’s Feb. 4 veto of a United Nations Security Council resolution on Syria for the recent upsurge in violence, saying it gave a green light to Assad to crush the revolt without fear of intervention.