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.

Kerry: US not against Russian strikes, but Assad must go
United Nations, United States (AFP) Sept 30, 2015 - The United States does not oppose Russian air strikes in Syria if they target the Islamic State group, but Bashar al-Assad must step down, Secretary of State John Kerry said Wednesday.

Kerry was addressing the United Nations Security Council hours after Russian jets hit targets in Syria, apparently in support of Assad's battle against rebel groups threatening his hold on power.

"If Russia's recent actions and those now ongoing reflect a genuine commitment to defeat that organization then we are prepared to welcome those efforts," Kerry said.

US forces, he added, are ready to "de-conflict" with Russian forces -- communicate to avoid accidental encounters on the battlefield -- "thereby increasing the military pressure on ISIL."

"But we must not and will not be confused in our fight against ISIL with support for Assad," he insisted.

"Moreover we have also made clear that we would also have grave concerns should Russia strike areas where ISIL and Al-Qaeda affiliated targets are not operating," he said.

"Strikes of that kind would question Russia's real intentions in fighting ISIL or protecting the Assad regime," he warned.

Early reports of Russia's strikes on Wednesday suggested they hit areas where other opposition groups, seen by the United States and its allies as moderates, have been fighting Assad's forces.

"We have informed Russia that we are prepared to hold these de-confliction talks as early as possible: this week," Kerry said.

"But let me be clear, the United States and the coalition will continue our ongoing air operations as we have from the very beginning," he said, referring to the year-old US-led campaign.

"We have conducted a number of strikes against ISIL targets in Syria over the past 24 hours including just an hour ago, and these strikes will continue."