Washington, Dec 12 (IANS): US President Barack Obama has said his country has formally recognised Syria's opposition rebel coalition as the "legitimate representative" of the Syrians, BBC reported.

Obama said the emerging coalition had earned the right to represent the Syrian people, but issued a note of caution as well saying: "Obviously, with that recognition comes responsibilities."

"To make sure that they organise 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," the US president said.

Describing the US' move, already made by Britain and the EU, as "a big step", Obama told ABC News, the coalition was now "inclusive enough, is reflective and representative enough".

Earlier in the day, the US Treasury imposed sanctions on the jihadist Al Nusra Front in Syria, after the State Department blacklisted the group as a foreign terror outfit linked to Al Qaeda in Iraq, CNN reported.

The Treasury also slapped sanctions on two armed militia groups that operate under the control of the Syrian government -- Jaysh al-Sha'bi and Shabiha

The UN has estimated that nearly 40,000 have died since the violence began in March 2011.