By Daniel Bases and Arshad Mohammed

NEW YORK (Reuters) - French Foreign Minister Laurent Fabius said on Monday he saw no legal obstacle to attacking the Islamic State militant group in Syria even as he stressed France did not plan air strikes there.

"There is not, to our way of thinking, any legal impediment to responding to Islamic State attacks in Iraq as well as in Syria," Fabius told reporters on the sidelines of the annual U.N. General Assembly.

France last week became the first foreign government to join the United States by conducting air strikes in Iraq against the Islamic State, a militant Sunni Islamist group that has seized a third of Iraq and large swaths of territory across Syria.

At a think tank earlier in the day, Fabius stressed that his nation would not launch air strikes against IS militants based in Syria despite having bombed a suspected target of the group in northern Iraq last week.

His stance on the legal question appeared at variance with the view expressed by French President Francois Hollande, who last week stressed that France had acted in Iraq at the request of the Iraqi government but had no similar authority in Syria.

Islamic State has been blamed for a wave of sectarian violence, beheadings - including of two U.S. citizens and one Briton - and massacres of civilians.

Fabius reiterated France's position that it will continue to support the moderate opposition against Syrian President Bashar al-Assad, but that "the French president has said we do not have intention to do the same in Syria, I mean by air."

"I think it is possible to act. Therefore the question is not a question of legality, international legality. But, first, France cannot do everything. And second, we consider that to support the moderate opposition and to fight both Bashar and Daech (Islamic State) is a necessity," Fabius said.

France is the only western nation to have publicly admitted to arming Assad's opponents, although it has limited deliveries once Islamic State gained territory.

Fabius said Iran, which is negotiating with western powers over its nuclear ambitions, could play some role against IS rather than formally joining a coalition. He said the Iranian and French presidents could explore this in talks this week.

However, he stressed France wanted to keep the nuclear issue and the battle against IS entirely separate.

"Because of their geographical position and what they say, and attitude towards Daech, they can do something. Not in the coalition, in the narrow sense of this word, but more generally speaking," Fabius said.

Shi'ite Muslim-dominated Iran is a key ally of the governments in Iraq and Syria.

Iran and the United States, Britain, Germany, France, Russia and China resume talks in New York this week on the sidelines of the United Nations General Assembly meeting in a bid to secure a long-term deal to curb Tehran's nuclear programme.

(Additional reporting by John Irish in Paris; Editing by Peter Galloway and Lisa Shumaker)