Russian Foreign Minister Sergei Lavrov has defended his country's air strikes in Syria, saying they were targeting the same "terrorist" groups as the US-led coalition.
He rejected suggestions that Russian action was to bolster Syrian President Bashar al-Assad.
Russian jets carried out a second day of air strikes in Syria on Thursday.
But a US official said Russia had been isolated by its "indiscriminate" strikes against the Syrian opposition.
White House spokesman Josh Earnest said that if Russia was genuinely focused on fighting Islamic State (IS) it would contribute to the US-led coalition.
The strikes risk drawing Russia more deeply into the conflict, he added.
The Russian defence ministry said five sites were attacked on Thursday and an IS command post at Jisr al-Shughour was destroyed.
The Kremlin says it is acting on the request of the Syrian government to help combat IS militants - who have seized parts of Syria and Iraq - and other designated terrorist groups.
But the US and its allies fear the strikes have mainly targeted opponents of Russia's ally, President Assad.
Mr Lavrov, speaking at the UN in New York, said Russia would fight IS and other terrorist groups including the al-Nusra Front - an al-Qaeda affiliate.
He said this position was the same as the US-led coalition which has been carrying out air strikes in Iraq and Syria for several months.
"We are not supporting anyone against their own people. We fight terrorism," he said.
"As far as I understand it, the [US-led] coalition announced Isil (IS) and other associated groups as the enemy. And the coalition does the same as Russia does."
Analysis by Jonathan Marcus, BBC diplomatic correspondent
The Syrian government's army may not be what it was, but in local terms it is still a force to be reckoned with.
Bolstered with new Russian equipment and now backed by Russian air power, it could hold its own against most of the opposition forces.
Russia does not have the elaborate intelligence gathering panoply of the US. But much of its targeting will be based upon tactical intelligence obtained from Syrian units on the ground.
This then is the key to Russia's strategy. It is to consolidate the Assad regime, to relieve the pressure points and to ensure that its ally remains a factor in any future diplomatic settlement.
To this end - and there are strong indications of this even from Russia's initial air strikes - Moscow will hit any opponents of the Syrian regime where necessary.
Mr Lavrov said the sites being attacked were selected "in co-ordination with the Syrian army".
On the Free Syrian Army he added: "We believe that the Free Syrian Army should be part of the political process like some other armed groups on the ground composed of Syrian patriotic opposition individuals."
Mr Lavrov also said there were no plans to expand air strikes to Iraq.
"We were not invited; we were not asked. And we are polite people, as you know. We don't come if not invited," he said.
Iran - also an ally of President Assad - called the Russian operation "a step toward fighting terrorism and toward resolving the current crisis".
The Russian air strikes have raised fears of accidental conflict with the US-led campaign.
But Mr Earnest said US and Russian officials had held hour-long talks to discuss "deconfliction" operations, and that this was the first of a series of discussions.
The Syrian civil war began with an uprising against the government but has since splintered into various rebel groups fighting President Assad's forces and also each other.
According to reports from activists and other sources, Russia's latest air strikes hit sites in the north-west held by a rebel alliance called the Army of Conquest, as well as areas in Homs and Hama provinces.
The strategic town of Jisr al-Shughour was hit, as well as areas in Idlib province and Hama province further south, according to Lebanon's al-Mayadeen TV.
Rebel activists also reported strikes at Ghantu in Homs province, close to some of Wednesday's attacks.
The commander of a US-trained rebel group said one of its training camps had been hit by two Russian sorties on Thursday.
Hassan Haj Ali, of the Liwa Suqour al-Jabal group, told Reuters news agency that about 20 missiles had hit their camp in Idlib province.
He said the Russian jets were identified by former Syrian air force pilots who are now members of his group.
The Russian defence ministry said civilian areas were not targeted, although the Syrian opposition says the strikes have killed civilians.
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.