President Obama formally recognized a newly formed Syrian opposition group as the legitimate representative of the Syrian people Tuesday, a move the administration hopes will speed the downfall of President Bashar al-Assad and the formation of an alternative government.

“We’ve made a decision that the Syrian Opposition Coalition is now inclusive enough, is reflective and representative enough of the Syrian population that we consider them the legitimate representative of the Syrian people in opposition to the Assad regime,” Obama said in an interview with ABC’s Barbara Walters scheduled for broadcast later this week.

The widely anticipated decision follows similar recognition by France, Britain and others, and it comes on the eve of a meeting of the Friends of Syria group of nations in Morocco on Wednesday that is expected to formally anoint the group. Secretary of State Hillary Rodham Clinton, who was to announce the recognition at the meeting, canceled her trip because of illness.

“It’s a big step,” Obama said.

As rebel fighters have made significant gains on the ground against Assad’s forces in recent weeks, the administration and its allies have become concerned that events on the ground, including the growing prominence of extremist groups among the rebel forces, will outpace efforts to organize a cohesive political opposition. The coalition was formed last month, under the tutelage of the United States and the government of Qatar, joining representatives brought from inside the 20-month-old Syrian uprising with those working to build support outside the country.

A look at the Syrian uprising nearly two years later. Thousands of Syrians have died and President Bashar al-Assad remains in power, despite numerous calls by the international community for him to step down.

“Not everybody who’s participating on the ground in fighting Assad are people who we are comfortable with,” Obama told Walters. “There are some who, I think, have adopted an extremist agenda, an anti-U.S. agenda, and we are going to make clear to distinguish between those elements.”

Obama specifically mentioned Jabhat al-Nusra, a militant group the administration on Tuesday designated as a terrorist alias of al-Qaeda in Iraq.

“Obviously, with that recognition comes responsibilities,” Obama said of the Syrian Opposition Coalition. “To make sure that they organize themselves effectively, that they are representative of all the parties, [and] that they commit themselves to a political transition that respects women’s rights and minority rights.”

Formal recognition of the opposition coalition evokes a similar move in Libya, when the United States, much of Europe and Arab states recognized opposition political leaders as part of an escalation of involvement in that country’s upheaval that led to military intervention. U.S. officials have repeatedly said that although all options remain open, administration policy for the moment is to provide only non-lethal assistance to the Syrian opposition.

As fighting continued, dozens of civilians were killed or injured in an attack in the central Syrian province of Hama, according to activists.

Details of the attack and the exact number of casualties in Aqrab, a village about 20 miles southwest of Hama, the provincial capital, were not clear. The Syrian Observatory for Human Rights, a watchdog group based in Britain, said that at least 125 people had been killed or wounded, many of them minority Alawites.

After nearly two years of unrest, the war in Syria has increasingly become a sectarian conflict as a predominantly Sunni Muslim opposition battles a government led by Alawites, whose religion is an offshoot of Shiite Islam. Rami Abdulrahman, the director of the Syrian Observatory, who uses a pseudonym, said it was possible that rebels had targeted the Alawites, but other activists blamed pro-government shabiha militants for the attack.

Interactive: Recent events in Syria

The killings come a day after rebels took over a large military base 15 miles west of Aleppo, the country’s commercial capital. At least 35 soldiers were killed in the heavy fighting and more than 60 were injured and captured by rebels, according to activists.

Rebel forces have taken over at least half a dozen military bases during the past two weeks, capturing new and heavier weapons at each stop and seemingly building momentum toward a decisive showdown with Assad’s forces.

After reports early this month that Syria was moving its chemical weapons stockpiles, Defense Secretary Leon E. Panetta said Tuesday that U.S. officials have not seen additional evidence in recent days to suggest that Assad is planning to use chemical weapons to blunt rebel advances.

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

The rebel victories have been spurred in part by the fighting prowess of religious extremist groups. The rebel force that took over the base Monday included fighters from the Free Syrian Army, the main rebel military coalition, as well as Jabhat al-Nusra, according to rebel fighters and activists.

The State and Treasury departments announced that they had amended the terrorist designation of the group al-Qaeda in Iraq to include Jabhat al-Nusra, also known as al-Nusra Front. A senior administration official, one of several who briefed reporters on the condition of anonymity, said the group was responsible for “hundreds of attacks, nearly 600, in major city centers across Syria” and the summary execution of prisoners.

“Al-Nusra Front has sought to portray itself as part of a legitimate Syrian opposition,” the official said. “But today’s actions are intended to expose them and make clear that the United States believes that al-Nusra’s extremist ideology has no role in a post-Assad Syria.

Officials described Jabhat al-Nusra as under the control of the Sunni fighters of al-Qaeda in Iraq, who had fought U.S. forces, and charged that it has infiltrated the moderate Syrian opposition to impose an extremist ideology on a post-Assad Syria. The broader opposition Free Syrian Army “itself has understood the threat that al-Nusra represents to the political transition in Syria” and excluded the group from a meeting last week in Turkey where the disparate rebel military set up a unified command, the official said.

The terrorist designation bans U.S. citizens from any financial dealings with the group, and another senior administration official said the administration would continue “to strive to ensure that our assistance doesn’t fall into the wrong hands.” While the United States provides only non-lethal aid to the Syrian opposition, Persian Gulf states provide both weapons and money, at least some of which is thought to be directed at Jabhat al-Nusra. “We absolutely have made our views known . . . to our international partners,” the first official said.

The Treasury Department also targeted sanctions against two Jabhat al-Nusra leaders, and the State Department announced sanctions against two pro-Assad militias, the shabiha and Jaysh al-Sha’bi, and their leaders.

A video posted online shows a fighter carrying a black flag often hoisted by al-Qaeda sympathizers as he walks across the military base in Aleppo province examining abandoned hardware. One fighter speaking on the video says that Jabhat al-Nusra is responsible for the victory along with the Shura Council of the Mujaheddin, another religious extremist group, and the mujaheddin brigade.

Despite recent tensions between Jabhat al-Nusra and more-secular groups, it appears that they have put aside their differences for the moment. The U.S. terrorist designation has hardly changed the standing of Jabhat al-Nusra among fighters on the ground.

“Until now, all al-Nusra has done is defend the people who have been victims of the regime’s aggressive acts,” said Yousef Sadeq, a Free Syrian Army fighter in Aleppo province who took part in the battle for the base and fought alongside fighters from the group. “The United States cannot consider them terrorists only because of what they believe in and because their ideas are close to al-Qaeda,” he said by Skype. “The United States should fight the Assad terrorism instead.”

Tuesday’s killings in Aqrab were preceded by a tense showdown between pro-government shabiha and fighters from the Free Syrian Army, according to the Syrian Revolution General Commission, an opposition group.

When the shabiha heard that an FSA unit was coming to confront them, they took dozens of hostages from the village, which has a mixed population of Alawites and Sunnis, and hunkered down in a building, according to activists.

A group of sheiks, or village elders, went to the location to try to convince the shabiha to release the civilians, but the sheiks themselves were kidnapped and subsequently killed by the shabiha, along with several of the hostages.

In a video posted online Tuesday, a teenager who others said was injured in the attack said he is Alawite and blamed the deaths on the shabiha.

“They gathered us all and told us to stay here. They forced us in. Then they started shooting at us and told us none of you will ever leave here alive,” he says, as a medic treats a wound on his right leg.

Dehghanpisheh reported from Beirut. Suzan Haidamous and Ahmed Ramadan in Beirut and Ernesto Londoño in Kuwait City contributed to this report.