Washington (AFP) - Russia's air strikes in Syria on Wednesday targeted opposition forces and not Islamic State jihadists, a US defense official said, contradicting Russian claims.

At the United Nations in New York, Secretary of State John Kerry made clear that Washington would have "grave concerns" should Moscow opt to strike targets in areas where IS fighters and Al-Qaeda-linked groups are not operating.

"We have not seen any strikes against ISIL, what we have seen is strikes against (the) Syrian opposition," the official said, referring to the Islamic State group.

The Russian defense ministry had earlier in the day said its strikes targeted IS militants.

The defense official, speaking on condition of anonymity, said the targets were the same ones Russian reconnaissance craft had flown over in recent days.

"They are striking areas that they've looked at over the past week or so," the official said.

But at the White House, spokesman Josh Earnest said it was "too early" for him to say "what target they were aiming at and what targets were hit."

Russian warplanes carried out strikes in Homs province, US officials said, while a Syrian security source and state media said Hama province was also hit.

The Pentagon has repeatedly warned of the need for "deconfliction" to ensure coalition and Russian planes don't inadvertently cross paths.

Russia only gave the United States an hour's warning ahead of the strikes and did not specify where they would occur, riling many in the Pentagon who had been hoping for clearer and more detailed lines of communication.