Russian fighter jets launched strikes on rebel-controlled areas in Syria on Wednesday, Sky News quoted Syrian opposition sources as saying.
According to the report, the Russian aircraft attacked in the vicinity of the cities of Homs and Hama, starting at midday.
Earlier on Wednesday, Russian President Vladimir Putin's decision to launch air strikes against Islamic State militants in Syria was unanimously backed by the parliament in Moscow.
The move, which sets the stage for Russia's biggest play in the region since the 1991 collapse of the Soviet Union, was announced as Syrian government warplanes conducted heavy strikes in Homs province and the United States and its allies struck Islamic State targets.
A French diplomatic source said on Wednesday that Russia's airstrikes appeared to be directed against Syrian rebels and not the Islamic State.
As part of its preparations, Moscow has already sent military experts to a recently established command center in Baghdad which is coordinating air strikes and ground troops in Syria, a Russian official told Reuters on Wednesday.
The Russian Defense Ministry said the center is used to share information on possible air strikes in Syria.
Limited operation with no boots on the ground
Sergei Ivanov, the Kremlin's Chief of Staff said Russia's missions would be limited and not open-ended and precluded the use of ground troops.
"As our president has already said, the use of ground troops has been ruled out. The military aim of our operations will be exclusively to provide air support to Syrian government forces in their struggle against ISIS," he said.
The decision to get involved militarily in Syria will be a further challenge for Moscow, which is already intervening in Ukraine at a time when its economy is suffering from low oil prices and Western sanctions.
A US-led coalition has already been bombing Islamic State in Iraq and Syria, but Russia has been highly critical, saying it has only yielded meager results so far. France announced at the weekend that it had launched its first air strikes in Syria.
Ivanov said the upper house of parliament had backed military action by 162 votes to zero after President Bashar Assad had asked for Russian military assistance.
The Syrian presidency confirmed that in a statement, saying Assad had written to Putin and Russia was increasing its military support as a direct result of that appeal.
Ivanov said Russia was only acting to protect its own interests in Syria, where it maintains a Soviet-era naval facility at Tartous, its only access to the Mediterranean.
"We're talking specifically about Syria and we are not talking about achieving foreign policy goals or about satisfying our ambitions ... but exclusively about the national interests of the Russian Federation," said Ivanov.
Russian military action would not be open-ended, he added, declining to say which aircraft would be used and when.
"The operations of the Russian air force can not of course go on indefinitely and will be subject to clearly prescribed time frames."
Russia's decision to intervene in Syria was prompted by a panicky realization that the Syrian government was being turned over on the battlefield, diplomats have told Reuters.
When it saw several months ago that Syrian government forces were retreating on several fronts at a rate that threatened Assad, its closest Middle East ally, the Kremlin quietly decided to despatch more men, weaponry and armour, diplomats and analysts told Reuters.
Defense Secretary Ash Carter said Wednesday that the Russian airstrikes in Syria appear to have targeted areas that do not include Islamic State fighters, a development which Secretary of State John Kerry said would cause "grave concern" for the United States.
Kerry told the United Nations Security Council that the U.S. would not object to Russians hitting Islamic State or al-Qaida targets but airstrikes just to strengthen the hand of Syrian President Bashar Assad would be worrisome.
"It does appear they were in in areas where there probably were not ISIL forces," Carter said of the Russian airstrikes, using an alternative acronym for Islamic State. He would not go into more detail.
Putin's spokesman said the vote by the Federation Council, Russia's upper house of parliament, meant Moscow would be practically the only country in Syria to be conducting operations "on a legitimate basis" and at the request of "the legitimate president of Syria".
The last time the Russian parliament granted Putin the right to deploy troops abroad, a technical requirement under Russian law, Moscow seized Crimea from Ukraine last year.
Analysts said Putin needed to get parliament's backing to ensure that any military operation was legal under the terms of the Russian constitution.
"If there will be a united coalition which I doubt, or in the end two coalitions -- one American and one Russian -- they will have to coordinate their actions," Ivan Konovalov, a military expert, told Reuters.
"For Russian forces to operate there legitimately ... a law was needed."
Reuters and the Associated Press contributed to this article.