The Obama administration has declared one of the Syrian resistance groups an al-Qaida front, as part of a gradual move by the US towards recognition of more moderate elements of the opposition.

The State Department said the al-Nusra Front for the People of the Levant, which is taking part in the fight on the ground against president Bashar al-Assad, is an alias for al-Qaida in Iraq (AQI), and designated it as a "foreign terrorist organisation". The Obama administration said that AQI has been supplying money, weapons and manpower to the al-Nusra Front.

Sanctions imposed as a result of the declaration against the al-Nusra Front will have almost no practical impact, other than to make travel for senior members of the group more difficult. The move is primarily diplomatic, aimed at isolating the group from what the Obama administration views as the more tolerant parts of the Syrian resistance.

The move comes the day before an international conference in Morocco at which the US is expected to take further steps towards the eventual recognition of the Syrian opposition as the sole legitimate representative of the Syrian people, as other countries have done.

The Obama administration has faced criticism from Republicans and from foreign affairs specialists in Washington that it risks helping extremist groups inside the resistance that are hostile to the US.

An Obama administration official, speaking on condition of anonymity, said the transition to a post-Assad government in Syria is gathering speed, and the US did not want extremists dictating the shape of the transition.

He said the al-Nusra Front rejected the vision of the mainstream Syrian opposition groups of a tolerant society and free elections. "It is an extremist organisation that has to be isolated," the official said in a telephone conference call with reporters. He said the aim was to expose the role of a-Nusra amid concern that its influence was expanding. The official hoped that countries in the region supporting the fighters would take note.

In a statement issued Tuesday morning, State Department spokeswoman Victoria Nuland said: "Since November 2011, the al-Nusra Front has claimed nearly 600 attacks – ranging from more than 40 suicide attacks to small arms and improvised explosive device operations – in major city centres including Damascus, Aleppo, Hama, Dara'a, Homs, Idlib, and Deir al-Zor. During these attacks numerous innocent Syrians have been killed.

"Through these attacks, 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. AQI emir Abu Dua is in control of both AQI and al-Nusra."

Additional sanctions were also imposed against senior figures and groups inside the Assad government accused of being involved in repression.