Russian military jets have carried out airstrikes in Syria for the first time, targeting what Moscow said were Islamic State (IS) positions.
A US official and others cast doubt on that claim, saying the Russians appeared to be attacking opposition groups fighting Syrian government forces.
President Vladimir Putin sought to portray the airstrikes as a pre-emptive attack against the Islamic militants who have taken over large parts of Syria and Iraq.
Russia estimates at least 2,400 of its own citizens are already fighting alongside extremists in Syria and Iraq.
"If they (militants) succeed in Syria, they will return to their home country, and they will come to Russia, too," Mr Putin said in a televised speech at a government session.
The US and Russia both agree on the need to fight IS, also known as Isis or Isil, but are in dispute about what to do about Syrian President Bashar Assad's government.
At the UN General Assembly, President Barack Obama said the US and Russia could work together on a political transition, but only if Assad leaving power was the result. Mr Putin is Assad's most powerful backer.
The Russian airstrikes targeted positions, vehicles and warehouses that Moscow believes belong to IS militants, Russian Defence Ministry spokesman Igor Konashenkov told Russian news agencies.
A senior US official, however, said the airstrikes do not appear to be targeting IS, because the militants are not in the western part of the country, beyond Homs, where the strikes were directed.
It appears the strikes were directed against opposition groups fighting against Assad, according to the official.
Syrian state television quoted an unnamed military official as saying that Russian warplanes have targeted IS positions in central Syria, including the areas of Rastan and Talbiseh, and areas near the town of Salamiyeh in Hama province.
IS controls parts of Homs province, including the historic town of Palmyra. Homs also has positions run by al Qaida's affiliate in Syria, known as the Nusra Front. Both groups have fighters from the former Soviet Union including Chechens.
Genevieve Casagrande of the Institute of the Study of War, using an alternative acronym for Islamic State, said the airstrike on Talbisah, "did not hit Isis militants and rather resulted in a large number of civilian casualties".
"If confirmed, the airstrike would signal Russian intent to assist in the Syrian regime's war effort at large, rather than securing the regime's coastal heartland of Latakia and Tartous," she said.
US State Department spokesman John Kirby said that a Russian official in Baghdad informed US Embassy personnel that Russian military aircraft would shortly begin flying anti-IS missions over Syria.
The Russian official also asked that US aircraft avoid Syrian airspace during those missions Wednesday. Mr Kirby did not say whether the US agreed to that request.
The US-led coalition fighting IS will continue to fly missions over Iraq and Syria, Mr Kirby added.
The US said 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.
Russian lawmakers voted unanimously on Wednesday to allow Mr Putin to order the airstrikes in Syria, where Russia has deployed fighter jets and other weapons in recent weeks.
The Federation Council, the upper chamber of the Russian parliament, discussed Mr Putin's request for the authorisation behind closed doors, cutting off its live web broadcast to hold a debate notable for its quickness.
Mr Putin had to request parliamentary approval for any use of Russian troops abroad, according to the constitution. The last time he did so was before Russia annexed Ukraine's Crimean Peninsula in March 2014.
Mr Putin insisted that Russia is not going to send ground troops to Syria and that its role in Syrian army operations will be limited.
"We certainly are not going to plunge head-on into this conflict," he said. "First, we will be supporting the Syrian army purely in its legitimate fight with terrorist groups. Second, this will be air support without any participation in the ground operations."
Mr Putin also said he expects Assad, Russia's long-time ally, to sit down and talk with the Syrian opposition about a political settlement, but added he was referring to what he described as a "healthy" opposition group.
Russia's first airstrike on Syria came after Mr Putin's meeting on Monday with US president Barack Obama on the sidelines of the UN General Assembly meeting in New York, where the two discussed Russia's military build-up in Syria.
Mr Putin and other officials have said Russia was providing weapons and training to Assad's army to help it combat IS. Russian navy transport vessels have been shuttling back and forth for weeks to ferry troops, weapons and supplies to an air base near the Syrian coastal city of Latakia. IHS Jane's, a leading defence research group, said last week that satellite images of the base showed 28 jets, including Su-30 multirole fighters, Su-25 ground attack jets, Su-24 bombers and possibly Ka-52 helicopter gunships.
Sergei Ivanov, chief of Mr Putin's administration, said in televised remarks after the parliamentary vote that Moscow was responding to a request from Assad asking for help.
He said the biggest difference between Russian airstrikes and those being conducted by the US and other countries is that "they do not comply with international law, but we do".
Moscow has always been a top ally of Assad. The war in Syria against his regime, which began in 2011, has left at least 250,000 dead and forced millions to flee the country. It is also the driving force behind the record-breaking number of asylum-seekers fleeing to Europe this year.
Worried by the threat of Russian and US jets clashing inadvertently over Syrian skies, Washington agreed to talk to Moscow on how to "deconflict" their military actions. Last week, US defence secretary Ash Carter had a 50-minute phone call with his Russian counterpart - the first such military-to-military discussion between the two countries in more than a year.
On Tuesday, Pentagon press secretary Peter Cook said Mr Carter had instructed his staff to contact Russian officials about establishing talks on ways to keep each other's air operations in Syria from colliding or otherwise getting in each other's way. Mr Cook said it was not yet clear when these talks would start.
Israel has taken similar precautions, with Prime Minister Benjamin Netanyahu visiting Moscow last week to agree with Mr Putin on a co-ordination mechanism to avoid any possible confrontation between Israeli and Russian forces in Syria.
Russian Federation Council chairwoman Valentina Matvienko said in a live news conference on Russian television that parliament's decision reflected Russia's growing role in global affairs.
"We as a great power cannot but take part in fighting this great evil," Ms Matvienko said, adding that the Soviet Union and Syria signed a security co-operation agreement in 1980 that guarantees that Moscow would help Damascus if asked. "We couldn't refuse Bashar Assad and keep on seeing how people, women and children are dying," she said.
In Baghdad, Saad al-Hadithi, a spokesman for the Iraqi prime minister, said his government was in talks with Russia "in the hope that shared intelligence will further our abilities to defeat the terrorists within our borders".