Spokesman Josh Earnest said Russia was carrying out random air strikes against the Syrian opposition.

Earlier, Russian Foreign Minister Sergei Lavrov said Russia was targeting the same terror groups as the US-led coalition.

He rejected suggestions that Moscow was trying to bolster the Syrian military.

Russia has said it is targeting Islamic State (IS) and other militant groups but the Syrian opposition and others have suggested rebel factions opposed to Syrian President Bashar al-Assad - the Kremlin's ally - are bearing the brunt of the attacks.

The Syrian military has lost ground to IS and various other rebel groups in recent months.

Paris talks will see Syria focus

Meanwhile, talks in Paris on Friday involving Russian President Vladimir Putin and hosted by his French counterpart Francois Hollande are expected to be overshadowed by the situation in Syria, although the meeting was called to discuss peace efforts in Ukraine.

"Carrying out indiscriminate military operations against the Syrian opposition is dangerous for Russia," said Mr Earnest in Washington.

"It is only going to prolong the sectarian conflict inside Syria, if not make that conflict indefinite, and it also risks Russia being drawn even more deeply into that conflict."

Mr Earnest said US and Russian officials had held hour-long talks on Thursday to discuss "deconfliction" operations, and that this was the first of a series of discussions.

Russia said it carried out its second day of air strikes on Thursday, and had hit IS ammunition dumps and command centres.

Mr Lavrov, speaking at the UN in New York, said Russia would also fight other terrorist groups including the al-Nusra Front - an al-Qaeda affiliate.

He said this position was the same as that of the US-led coalition which has been carrying out air strikes in Iraq and Syria for the past year.

"We are not supporting anyone against their own people. We fight terrorism," he said.

"As far as I understand it, the [US-led] coalition announced Isil (IS) and other associated groups as the enemy. And the coalition does the same as Russia does."

Mr Lavrov said the targets were selected "in co-ordination with the Syrian army".

However, he said Russia did not consider the opposition Free Syrian Army a terrorist group and said it should be part of the political process.

Mr Lavrov also said there were no plans to expand air strikes to Iraq.

Iran - also an ally of President Assad - called the Russian operation "a step toward fighting terrorism and toward resolving the current crisis".

The Syrian civil war began with an uprising against the government but has since splintered into various rebel groups fighting President Assad's forces and also each other.

Russian air strikes on Thursday were said to have hit Homs, Hama and Idlib provinces.

The commander of a US-trained rebel group said one of its training camps in Idlib province had been hit by two Russian sorties.

Hassan Haj Ali, of the Liwa Suqour al-Jabal group, told Reuters news agency that the Russian jets were identified by former Syrian air force pilots who are now members of his group.

Syrian opposition groups says the strikes have killed civilians but Russian officials say the warplanes have avoided civilian areas.

Syria's civil war

What's the human cost?

More than 250,000 Syrians have been killed and a million injured in four-and-a-half years of armed conflict, which began with anti-government protests before escalating into a full-scale civil war.

And the survivors?

More than 11 million others have been forced from their homes, four million of them abroad, as forces loyal to President Assad and those opposed to his rule battle each other - as well as jihadist militants from IS and other groups. Growing numbers of refugees are going to Europe.

How has the world reacted?

Regional and world powers have also been drawn into the conflict. Iran and Russia, along with Lebanon's Hezbollah movement, are propping up the Alawite-led government. Turkey, Saudi Arabia and Qatar are backing the Sunni-dominated opposition, along with the US, UK and France.

The battle for Syria and Iraq in maps

(BBC)