Russian president Vladimir Putin says his air force will be supporting the Syrian army in its offensive operations.
Mr Putin said that Russia is "not going to plunge into this conflict head-on" and will help Bashar Assad's army as long as their offensive operation lasts.
Mr Putin also said he expects President Assad to sit down and talk with the Syrian opposition about a political settlement in Syria.
The upper chamber of Russian parliament earlier gave the green light to Mr Putin's request to use Russian armed forces in Syria.
Russian defence ministry spokesman Igor Konashenkov told Russian news agencies that jets are carrying out targeted airstrikes on the positions, vehicles and warehouses that Russia believes belong to IS militants.
President Assad welcomed the decision by Russia to send troops to his war-torn country, saying the military support from Moscow is the result of a Damascus request.
According to a statement by Mr Assad's office, the Syrian leader had sent a letter to his Russian counterpart, asking for the support. Mr Assad's Facebook page also reiterated that it "came upon a request from the Syrian state".
Russia has been one of President Assad's strongest allies since the Syrian crisis began in March 2011. The civil war has killed more than 250,000 people and wounded a million, according to UN figures.
On the ground, Syrian activists said that air raids on the central provinces of Homs and Hama killed and wounded dozens of people on Wednesday.
An activist group known as the Local Coordination Committees claimed that the planes that carried out the air raids were Russian.
It cited residents in the areas bombed as saying the explosions were much more powerful and accurate than those carried out by government planes.
A senior US official later said that Russia's airstrikes in Syria did not appear to be targeting the Islamic State group, but other opposition groups fighting against President Assad.
The official said IS militants are not in the western part of the country, beyond Homs, where the Russian strikes were directed.
Russia notified the US about the impending airstrikes through the embassy in Baghdad about an hour before they started, the official said.
According to the official, there were no conflicts with the Russian strikes, and they had no impact on the coalition missions, which are primarily in the north and east.
The US says it is still trying to assess the damages of the Russian strikes.
