* Jabhat al-Nusra considered 'terrorist organization' -U.S.
* Group linked to al Qaeda in Iraq, backs Islamic state
* U.S. action freezes group's assets, blocks other support
* U.S. also targets Syrian militias working under Assad (Adds details, quotes)
By Susan Heavey and Andrew Quinn
WASHINGTON, Dec 11 (Reuters) - The United States on Tuesday designated the radical Islamist Syrian rebel group Jabhat al-Nusra, an important fighting force in the opposition struggle, as a foreign terrorist organization and said it was trying to hijack the rebellion on behalf of al Qaeda in Iraq.
U.S. officials said the al-Nusra group had claimed responsibility for carrying out nearly 600 attacks in major cities that have killed numerous innocent Syrians during the 20-month uprising against President Bashar al-Assad.
"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," U.S. State Department spokeswoman Victoria Nuland, using the acronym for al Qaeda in Iraq, said in a statement.
The U.S. Treasury, in a parallel move, also slapped sanctions on two militia groups working for the Assad government, including the infamous Shabiha paramilitary organization accused of atrocities against civilians.
Designating al-Nusra as a terrorist group means U.S. authorities can freeze any assets the group or its members have in U.S. jurisdictions. It also prohibits Americans from giving it any material support.
U.S. officials conceded that the action was unlikely to immediately curtail al-Nusra's activities, but said it was an important signal both to the Syrian opposition and its foreign supporters, particularly in the Gulf, that al-Nusra and its ilk cannot play a part in Syria's eventual political transition.
"I think that other nations that are involved in helping the armed opposition will now take more seriously our concerns about the Nusra front and its expanding influence," one senior U.S. official told reporters in a briefing on the move.
"It is important for countries to understand what al-Nusra is and what it represents."
Tuesday's action came as U.S. officials attend the Friends of Syria meeting in Marrakech, Morocco, to discuss the Syria crisis, as rebels push forward on the battlefield and move to unify the political opposition.
Al-Nusra, one of the most efficient fighting groups in Syria, is not part of a new, Islamist-dominated military unified rebel command elected in Turkey over the weekend.
Some other groups value it for its fighting prowess while others accuse it of indiscriminate tactics, such as poorly placed car bombs that have harm civilians, and summary executions.
"They have their own leaders and their own structure, they fight side-by-side with the Free Syrian Army. We have only seen good things from them and they are good fighters," said Abdul Jabbar al-Oqaidi, a senior commander in the new group.
Its actions have included more than 40 suicide attacks as well as the use of small arms and improvised explosive device operations and have targeted major city centers, including Damascus, Aleppo, Hamah and Dara, U.S. officials said.
Some activist and rebel groups criticized the U.S. moves against al-Nusra. Nearly 30 activist and rebel groups have called for protests on Friday to support the group.
"Jabhat al-Nusra is one of the strongest forces fighting to get rid of the regime ... I have never seen any attacks that could be considered a violation of civilians' rights. There has never been any proof of this," Samir al-Shami, an opposition activist in Damascus with the Syrian Youth Union told Reuters.
Musaab Abu Qitada of the rebel group the Damascus Military Council, said groups like al-Nusra "have helped defend our children and have sacrificed their own lives for our families."
But another activist who lives in Hama, who asked not to be named over concerns for his safety, welcomed the U.S. action but said such groups are gaining a foothold in the absence of any other real military support.
"I don't believe most Syrians want these radical groups. We are a conservative people but not extremists. I think Syrians have accepted them because they feel they have no other choice," the activist said.
U.S. officials said the designation of al-Nusra was not aimed at crippling the broader military struggle against Assad, emphasizing that it was a small if influential component of the fighting force arrayed against Damascus.
But they indicated that the Obama administration, which has thus far limited its aid to the Syrian opposition to non-lethal assistance such as medical and communications equipment, was still leery of steps to directly arm anti-Assad fighters.
"For us, providing arms has to be done in a way that helps promote a political solution, and until we understand how we are going to promote a political solution we do not see how provision of arms is a good idea," one official said.
Separately, the Treasury Department moved to block the property of the Syrian government by sanctioning two militia groups that work under the Assad government, Jaysh al-Sha'bi and Shabiha.
It also took action against two commanders of the Shabiha group: Ayman Jaber, who it said was working with the Assad forces to obtain weapons for the militia, and Mohammad Jaber, who helped transport Shabiha members to Turkey to attack Syrian rebels in the neighboring country.
The Treasury Department, in its statement, said the militias it is targeting are part of the Assad regime's campaign against Syrian citizens. Jaysh al-Sha'bi has ties to Iran and Hezbollah, it said. (Reporting By Susan Heavey, Mark Hosenball, Andrew Quinn and Mohammad Arshad; additional reporting by Erika Solomon in Beirut; Editing by David Storey and Philip Barbara)