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 targeted the Islamic State group, but an
unnamed US official told Reuters that so far they did not appear to be
targeting IS-held territory.
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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".
RUSSIAN DEFENCE MINISTRY Su-24 fighter-bomber aircraft are said to have been involved in the strikes
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.
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
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.
