United States President Barack Obama has recognised the Syrian opposition as the legitimate representative of the Syrian people.
The move follows recognition by European countries, Turkey and several Arab states.
A month ago, Syrian opposition factions meeting in Qatar agreed to form a unified political coalition to topple President Bashar al-Assad.
In an interview on American television, Mr Obama said the National Coalition was now inclusive, reflective and representative enough for the US to take this "big step", the BBC reports.
Britain, France, Turkey and Gulf states have already given their recognition.
Russia says the US has decided to place all its bets on the coalition achieving an "armed victory".
Activists say more than 40,000 people have died in the 18-month uprising.
Reports from central Syria on Wednesday say a large number of civilians from the president's Alawite sect were killed in a village to the west of Hama, reports the BBC.
The circumstances are unclear but a video posted by opposition activists purported to show a young Alawite boy describing how government forces had destroyed a building where the civilians were being held hostage by pro-government militia.
The building, in the village of Aqrab, had been under siege from the Free Syrian Army and, according to the boy, as many as 300 civilians were killed,
There has been no word so far from the Syrian government and it is impossible to verify the activists' account.