Syria crisis: Russia and US military to hold talks on air strikes
The US and Russian military will hold talks "as soon as possible" to avoid clashing in Syria, the countries' top diplomats say.
Russian defence officials say their aircraft carried out about 20 missions against the so-called Islamic State group (IS) on Wednesday.
But the US expressed fears the targets were non-IS opponents of Russia's ally, Syrian President Bashar al-Assad.
The US is targeting IS with air strikes in both Syria and Iraq.
Nato said there had been little co-ordination by Russia with US-led forces against IS, also known as Isil. The US says it was informed of Wednesday's air strikes only an hour before they took place.
Analysis: Jon Sopel, BBC North America editor
Are the Russians trying to destroy IS or are they trying to prop up President Assad? The Americans see the two as very different, the Russians less so.
On the evidence of one day's bombing, it looks to be very much the latter. The target of Russian ordnance has been in places where IS has no presence.
In other words, other rebel groups - any rebel groups - opposed to President Assad are being targeted. And that raises another huge question. What if the Russians start hitting US-backed and armed rebel groups? That is not going to end well.
Russia's Foreign Minister Sergei Lavrov said there was a need to "establish channels of communication to avoid any unintended incidents". His US counterpart, John Kerry, said talks will be held "as soon as possible," maybe as early as Thursday.
Mr Kerry added: "It's one thing to be targeting Isil, but the concern, obviously, is that this is not what was happening."
France's Defence Minister, Jean-Yves Le Drian, told French MPs: "Curiously, they didn't hit Islamic State. I will let you draw a certain number of conclusions yourselves."
US Defence Secretary Ashton Carter said Russia's approach was "tantamount to pouring gasoline on the fire" and "doomed to fail" because of the breadth of Syrian opposition to Mr Assad.
Syria's civil war has raged for four years, with an array of armed groups fighting to overthrow the government.
The US and its allies have insisted that President Assad should leave office, while Russia has backed him remaining in power.
The Russian defence ministry said the country's air force had targeted IS military equipment, communication facilities, arms depots, ammunition and fuel supplies - and did not hit civilian infrastructure or areas nearby.
But Syrian opposition activists said Russian warplanes hit towns including Zafaraneh, Rastan and Talbiseh, resulting in the deaths of at least 36 civilians, a number of them children.
None of the areas targeted was controlled by IS, activists said.
A resident of Talbiseh told the BBC that the Russians were striking the only area in that part of Syria that was still under rebel control. "In my opinion they are helping Assad to get rid of the rebels in general," he said.
One doctor in Rastan told Reuters: "We have been exposed to a wide range of weapons over the last five years. But what happened today was absolutely the most violent and ferocious, and the most comprehensive."
Russian President Vladimir Putin said Russia was acting against the threat of militants in Syria returning to their home countries, including Russia.
Saudi Arabia, a major backer of the rebels which has always insisted that Mr Assad must leave power, expressed "profound concern" at the Russian strikes.
The Saudi UN ambassador, Abdallah Al-Mouallimi, demanded that they stop immediately, adding that countries should not be fighting IS "at the same time as they support the terrorism of the Syrian regime".
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.