Government air strikes in towns of Talbiseh and Zaafaraneh in Homs leave at least 27 people, including children, dead.
Russian military jets have carried out air strikes in Syria for the first time, the defence ministry has said.
The air strikes targeted positions, vehicles and warehouses that Russia believes belong to the Islamic State of Iraq and the Levant (ISIL) group, ministry spokesman Igor Konashenkov told Russian news agencies.
US Secretary of State John Kerry complained to his Russian counterpart Sergei Lavrov after the strikes.
Speaking on condition of anonymity, the official said Kerry told Lavrov that the Russian move “runs counter to their stated efforts of deconfliction and is not helpful to that effort.”
Lavrov later told the UN Security Council that his country was “ready to forge standing channels of communication to ensure a maximally effective fight against terrorist groups” with the US and other countries.
Earlier on Wednesday, Russia’s upper house of parliament granted President Vladimir Putin authorisation to deploy the country’s air force to Syria, according to the head of presidential administration.
Sergey Ivanov said that the Federation Council backed Putin’s request for approval unanimously.
“The operation’s military goal is exclusively air support of the Syrian armed forces in their fight against ISIL,” he said.
He said Putin’s move followed a request for military help by Assad.
Ivanon said that the operation by the Russian air force was limited in time and the types of weapons used were not disclosed.
Putin’s request came on Wednesday, two days after he suggested at the UN General Assembly in New York teaming up with the US to carry out air strikes against ISIL in Syria, but ruled out any ground operation in the country.
Russia’s powerful Orthodox Church has reportedly voiced support for Moscow’s decision to carry out air strikes, telling the Russian Interfax news agency it was a “holy battle”.
Putin has to request parliamentary approval for any use of Russian troops abroad, according to the Russian constitution.
The last time he did so was before Russia annexed Ukraine’s Crimea Peninsula in March 2014.
The Kremlin reported that Putin hosted a meeting of the Russian security council at his residence on Tuesday night outside of Moscow, saying that they were discussing “terrorism and extremism”.
On Saturday, Federica Mogherini, the EU’s foreign policy chief, told Al Jazeera that Russia was intending to step up its military involvement in Syria to prevent the “imminent” collapse of the Syrian government.
Speaking on Talk to Al Jazeera, Mogherini said Russia’s foreign minister told her his country wanted to prevent the collapse of the Syrian state.
“His fear is of a complete collapse of the state structures in Syria; this could be one of the reasons Russia is talking in this way, but it could also be willingness to show that Russia is an important, substantial player,” Mogherini said.
Her comments followed reports that 500 Russian troops have already been deployed to a forward operating base in the Syrian port city of Latakia.
In another development, a Russian media report said that some Russian military contractors are being sent to Syria without their consent.
The report, published by the Russian online newspaper gazeta.ru on Friday, said that 20 soldiers, including officers and contracted soldiers, were selected from a military unit of Russia’s Eastern military district command and sent on a mission without being told about the final destination.
According to the contractors cited in the report, the commander only told them at the beginning that they might be sent to a “hot country”.
On September 16, they were informally told by a representative of a military chief about the secretly planned deployment to Latakia, prompting protests from “almost everyone”.