The U.S. Treasury imposed sanctions Tuesday on leaders of the jihadist al-Nusra Front in Syria, hours after the State Department moved to blacklist the rebel group as a foreign terror organization linked to al Qaeda in Iraq.
The Treasury also sanctioned two armed militia groups that operate under the control of the Syrian government, Jaysh al-Sha'bi and Shabiha, it said.
Syrian opposition groups have voiced their opposition to the U.S. move against the rebel fighters, suggesting that they are being targeted because they oppose a new anti-government coalition.
The move came the same day President Barack Obama recognized the leading Syrian opposition coalition as the legitimate representative of the country's people.
"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," he told ABC's Barbara Walters.
In recent months, the radical Islamist al-Nusra Front has emerged as one of the most effective groups in the Syrian resistance, drawing on foreign fighters with combat experience in Iraq and elsewhere.
But Washington accuses the group of using the Syrian conflict to advance its own ideology and ends.
State Department spokeswoman Victoria Nuland confirmed Tuesday that the al-Nusra Front had been added to the list of aliases for al Qaeda in Iraq, already designated a foreign terrorist organization.
She said the group had claimed nearly 600 attacks in several cities in the past year, including suicide bombings, and was responsible for the deaths of "numerous innocent Syrians."
Al-Nusra "has sought to portray itself as part of the legitimate Syrian opposition while it is, in fact, an attempt by AQI to hijack the struggles of the Syrian people for its own malign purposes," she said.
The designation makes it illegal for any U.S. citizen to give "material support or resources," including money, training or weapons, to al-Nusra fighters.
The Treasury's financial sanctions also target two senior al-Nusra leaders, named as Maysar Ali Musa Abdallah al-Juburi and Anas Hasan Khattab.
The measure means that any assets they may hold in the United States are blocked and that U.S. citizens are barred from doing business with them.
The Treasury sanctions against pro-government groups target two Shabiha commanders, named as Ayman Jaber and Mohammad Jaber, along with the two militia groups.
Its statement said the Shabiha have "operated as a direct action arm of the government of Syria and its security services," working alongside its intelligence services, and have been "complicit in the commission of human rights abuses in Syria, including those related to repression."
"Since the beginning of the unrest, the Shabiha have fired into crowds of peaceful Syrian demonstrators, shot and killed Syrian demonstrators, arbitrarily detained Syrian civilians, and shot Syrian soldiers who refused to fire on peaceful demonstrators," it said.
The Jaysh al-Sha'bi militia has "conducted unilateral and joint operations with Syrian military and security elements against the Syrian opposition" that have led to the deaths of opposition members, the statement said.
It accuses Iran of training, funding and arming the Jaysh al-Sha'bi militia.
Washington's move comes a day ahead of a Friends of Syria meeting scheduled in Morocco.
The goal of the designation is to isolate extremist groups in Syria while giving a boost to the new political opposition group unveiled last month in Doha, Qatar, U.S. officials said last week.