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.
The Russian Defence Ministry said it had carried out about 20 flights over Syria, hitting eight Islamic State targets.
The ministry said Russian jets had destroyed an Islamic State command post and an operations centres in a mountainous area, Russian agencies reported. The strikes did not hit civilian infrastructure or areas nearby, the ministry said.
The ministry said the country's air force had targeted IS military equipment, communication facilities, arms depots, ammunition and fuel supplies.
US officials said targets in the Homs area appeared to have been struck, but not areas held by Islamic State.
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.
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".
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.
A US defence official said: "A Russian official in Baghdad this morning informed US embassy personnel that Russian military aircraft would begin flying anti-Isil [IS] missions today over Syria. He further requested that US aircraft avoid Syrian airspace during these missions."
US state department spokesman John Kirby told reporters: "The US-led coalition will continue to fly missions over Iraq and Syria as planned and in support of our international mission to degrade and destroy Isil [IS]."
Syria's civil war
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.
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.
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.
- BBC / Reuters