BRUSSELS (Reuters) - NATO said on Wednesday it was concerned that Russia's air strikes on Syria may not have targeted Islamic State positions but hit areas held by Western-backed rebels.

Russia launched air strikes in Syria on Wednesday in the Kremlin's biggest Middle East intervention in decades, but Moscow's assertion that it had hit Islamic State militants was disputed by the United States and Western-backed rebels on the ground.

"I'm concerned about the reports saying that the Russian air strikes were not targeted against ISIL," NATO Secretary-General Jens Stoltenberg said during a visit to the United States.

While Russia says its raids targeted Islamic State, locals in the targeted area of Homs say the jihadist group has no presence in the region, echoing the assessment of a U.S. official and British-based Syrian Observatory for Human Rights.

"I'm especially concerned because there has been no real effort by the Russian side to deconflict the Russian air strikes in Syria with the ongoing US-led coalition fighting ISIL," he said, using an acronym for Islamic State.

To "deconflict", in military parlance, is to ensure that, in this case, Russian aircraft do not accidentally clash in any way with Western warplanes.

Stoltenberg said Russia had informed the Western alliance that Moscow was providing military assistance to Syrian President Bashar al-Assad. But he called on Russia to seek a diplomatic end to the Syrian civil war.

"I urge Russia to play a constructive and cooperative role in the fight against ISIL and to strive for a negotiated political solution to the conflict. To support Assad will not help," Stoltenberg said.

He also said NATO, which has been focussed on confronting a more assertive Russia since Moscow's annexation of Crimea, needed to be active to bring stability in the region, even if that did not mean deploying large numbers of combat forces.

"This is one of the main challenges we face as an alliance: the increased turmoil, violence, failing states, the terror which we see in the Middle East, North Africa," he said.

(Reporting by Robin Emmott; Editing by Ralph Boulton)