Al-Nusra and several other groups last month announced their opposition to the National Coalition for Syrian Revolutionary and Opposition Forces, a new anti-government coalition. U.S. officials estimate al-Nusra members represent about 9% of rebel forces in Syria.
The Syrian Observatory for Human Rights, a London-based opposition group, said fighters from the al-Nusra Front were among rebel forces who it says have seized control of a government military base in the Sheikh Sleiman area of western Reef Aleppo.
But their designation as a foreign terrorist group isn't being made on the grounds of past or possible future actions, according to the Observatory's Rami Abdelrahman.
"The United States decided to single out the Nusra Front because of their recent rejection to the political opposition front and (because) they have a different approach to post-Assad's Syria," he said.
Syrian Minister of Information Omran al-Zoubi told Lebanese al-Manar TV on Monday that Damascus understood why Washington wanted to blacklist the al-Nusra Front.
"When the U.S. places Jabhat al-Nusra on the international terrorist organizations list, that is because it realizes the nature of these groups which are fighting the Syrian armed forces," he said.
Syrian President Bashar al-Assad has characterized the nearly 21 months of violence that have ravaged his country as a fight against terrorism.
But the Syrian National Council, a largely expatriate opposition group, on Sunday voiced its "full rejection of any accusation of extremism and terrorism to any of the forces that are fighting the Syrian regime."
Any accusations made against factions within the Free Syrian Army, which brings together disparate groups, were intended to cause division within its ranks and between its forces and the Syrian people, it said.
"Terrorism is a characteristic that can only be attributed to the Syrian regime," it said.
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.