Washington blacklisted an al-Qaeda-linked rebel group in Syria warning extremists could play no role in building the nation's future as the US readies to recognize the new Syrian alliance.
The move against the al-Nusra Front came ahead of talks in Morocco on Wednesday, when the United States is expected to give full recognition to the Syrian National Coalition as the legitimate representative of the Syrian people.
Though a minority, al-Nusra has been one of the most effective rebel groups fighting to overthrow President Bashar al-Assad, raising concerns that hardline extremists are hijacking the 21-month-old revolt.
"What is important is to understand that extremists fighting the Assad regime are still extremists and they have no place in the political transition that will come," a senior State Department official said.
"Extremists should not dictate that political transition," he insisted on a conference call with journalists, asking to remain anonymous.
The State Department designated the group linked to al-Qaeda in Iraq (AQI) a foreign terrorist organization, while the Treasury also slapped sanctions on two of its leaders, Maysar Ali Musa Abdallah al-Juburi and Anas Hasan Khattab.
"Exposing the operation and the identities of al-Nusra's leaders is a key objective here," another top US official said.
Topping the agenda at the Friends of Syria meeting in Marrakech will be two key issues - the political transition after Assad's fall and mobilizing humanitarian aid as winter sets in amid a growing refugee crisis.
Declaring al-Nusra a terrorist group freezes its assets and bans Americans from any transactions with it, but US officials said it would also help ensure that vital aid is falling into the right hands.
Countries wanting to support the opposition need to ensure they are helping "those opposition groups who truly have the best interest of Syria and Syrians in mind," State Department spokeswoman Victoria Nuland said.
They should not back "groups coming from the outside who want to hijack what the Syrian people have started for their own means, and have a very different future in mind, a future that is based in al-Qaeda-based values and principles, not democratic-based principles and values."
The group has claimed responsibility for recent suicide bombings that killed scores of people, and has said it hopes to replace the Assad family's four-decade hold on power with a strict Islamic state.
Wednesday's talks could mark a step forward for the Syrian opposition, which had struggled for months to unite until a new coalition arose from November meetings in Qatar.
"Now that there is a new opposition formed, we are going to be doing what we can to support that opposition," US Secretary of State Hillary Clinton said in Brussels last week.
Clinton had planned to attend the Marrakesh meeting but canceled her trip on Monday due to illness. Deputy Secretary of State William Burns is traveling in her place.
She also met on her Europe trip with Russian Foreign Minister Sergei Lavrov and UN peace envoy Lakhdar Brahimi to see if there were ways to increase the pressure on Assad to step down and end the bloodshed.
Since the last Friends of Syria meeting in Paris in July, the number of people killed has risen from 16,000 to more than 42,000, the Britain-based Syrian Observatory for Human Rights says.
Al-Qaeda in Iraq has carried out scores of massive bombings aimed at Shiite civilians and regularly targeted US forces before their withdrawal a year ago.
The Treasury Department also sanctioned two armed militia groups supporting the Assad regime - Jaysh al-Sha'bi and Shabiha - as well as two Shabiha commanders.
The United States "will target the pro-Assad militias just as we will the terrorists who falsely cloak themselves in the flag of the legitimate opposition," said David Cohen, under secretary for terrorism and financial intelligence.
The Treasury Department said the militias have been "instrumental in the Asad regime's campaign of terror and violence against the citizens of Syria."
Nuland warned the US may target other groups. "There's a lot of morphing and changing within these militia groups," she said. "What we are saying is this is a false flag that we can identify and beware."