The commander of the Falcons of the Levant Brigade, a rebel group, criticized the U.S. move in a statement, saying the international community "should have designated Bashar al-Assad, his army and his criminal thugs on that list first and last for what they are committing against our people."

The group said it would "refuse to be dragged into these Western accusations against any group" and would continue to back al-Nusra and any other faction fighting government forces.

U.S. officials have previously said the jihadist al-Nusra Front has not affiliated itself publicly with al Qaeda in an apparent effort to appear more mainstream. The group has claimed responsibility for complex attacks in Damascus and Aleppo, frequently involving suicide bombers.

At least 165 people were killed Tuesday across the country, according to the opposition Local Coordination Committees of Syria.

The Syrian Observatory for Human Rights reported an attack in the village of Aqrab, in Hama province. More than 125 people were either killed or injured, the vast majority of which belong to the Alawite minority, it said. Al-Assad hails from the same group.

A local activist in Aqrab told CNN the village has been under heavy shelling for about a week. The shelling intensified Tuesday, forcing families to flee, said Hekam Abu Rayan. He described bodies still trapped under the rubble.

The Hama Revolutionary Command Council, an activist group affiliated with the Free Syrian Army, similarly reported deaths in Aqrab. It accused a pro-government militia of using women and children as "human shields," holding them in a building surrounded by rebel fighters. When rebels sent in people to negotiate with the militia, they were taken hostage and later killed, the council said. Women and children attempting to escape were also killed, it said.

Syrian state news, citing a military source, denied any attack in Aqrab.

CNN is unable to confirm casualty reports as the government has severely restricted access by international journalists.

The meeting in Morocco this week follows a renewed international push for a diplomatic solution to the crisis in Syria, amid concerns about the potential use of chemical weapons.

U.S. Defense Secretary Leon Panetta told reporters Tuesday, however, that intelligence about new attempts by Syrian government forces to move chemical weapons "has really kind of leveled off."

U.S. officials said last week that they had seen intelligence suggesting that Syrian military units might be preparing chemical weapons for use, prompting strong warnings from international figures.

"We haven't seen anything new indicating any aggressive steps to move forward in that way," Panetta said.

"But we continue to monitor it very closely, and we continue to make clear to them that they should not under any means make use of these chemical weapons against their own population. That would produce serious consequences."

Panetta said he would like to believe that al-Assad has gotten the message: "We've made it pretty clear and others have as well."

But, he added, "You know it's also clear that the opposition continues to make gains in Syria, and our concern is that if they feel like the regime is threatened with collapse that they might resort to these kinds of weapons."

President Obama has said that any use of chemical weapons would cross a "red line," eliciting a swift U.S. reaction.

Syrian state-run media said Monday that the United States has falsely accused Syria of considering the use of chemical weapons.