Russia has begun carrying out air strikes in Syria against opponents of President Bashar al-Assad.
The strikes reportedly hit rebel-controlled areas of Homs and Hama provinces, causing casualties.
The US says it was informed an hour before they took place.
Russian defence officials say aircraft carried out about 20 missions targeting Islamic State, but US officials said that so far they did not appear to be targeting IS-held territory.
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 its ally remaining in power.
The upper house of the Russian parliament earlier granted President Vladimir Putin permission to deploy the Russian air force in Syria.
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.
Syrian opposition activists said Russian warplanes had hit towns including Zafaraneh, Rastan and Talbiseh, resulting in the deaths of 36 people, a number of them children.
None of the areas targeted were controlled by IS, activists said.
US Secretary of State John Kerry said the United States was prepared to welcome Russian military action in Syria - but only as long as it was directed against IS and al-Qaeda-linked groups.
Speaking at the United Nations Security Council, Mr Kerry said the US would have "grave concerns" if Russia conducted strikes against other groups.
He said the US-led coalition against IS would "dramatically accelerate our efforts" and that the US was prepared to hold talks with Russia about avoiding accidental conflicts between the two air strike campaigns "as early as possible".
US Defence Secretary Ashton Carter said the Russian strikes were "probably not" in IS-controlled areas. He warned Russia's military moves were "doomed to fail".
Analysis: Jonathan Marcus, defence and diplomatic correspondent
Russia's decision to intervene with its air power greatly complicates the Syrian crisis while probably offering little additional chance of a diplomatic resolution.
Russian sources indicate that Sukhoi Su-24 warplanes were involved, operating out of an airbase near Latakia.
There are serious questions about who exactly the Russian aircraft are targeting. US officials believe that the initial Russian strikes are not in IS-held territory, raising the possibility that Russian air power is being utilised more in the form of close air support for Syrian government forces against the multiple enemies of the Assad regime.
Of course, many of these enemies are supported by the West's Arab allies or Turkey. The warning time given by the Russians to the Americans announcing the start of their operations may also raise some eyebrows, suggesting that much more detailed co-ordination may be needed in future to avoid incidents in Syrian airspace.
In a televised address, Mr Putin said the air strikes were targeting Islamist militants - including Russian citizens - who have taken over large parts of Syria and Iraq.
"If they [militants] succeed in Syria, they will return to their home country, and they will come to Russia, too," he said.
He added that Russia was not going to send ground troops to Syria, and that its role in Syrian army operations would be limited.
"We certainly are not going to plunge head-on into this conflict... we will be supporting the Syrian army purely in its legitimate fight with terrorist groups."
Mr Putin also said he expected President Assad to talk with the Syrian opposition about a political settlement, but clarified that he was referring to what he described as "healthy'' opposition groups.
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.
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