The White House said on Thursday it was open to limited talks with Moscow following the controversial deployment of Russian troops and heavy weapons to war-torn Syria.

White House spokesman Josh Earnest said President Barack Obama's administration was willing to hold "tactical, practical discussions" on operations in Syria and the fight against the Islamic State (IS) militant group.

The decision signals a newfound willingness to engage with Russia, after months of giving Vladimir Putin the cold shoulder over his actions in Ukraine and Syria.

Russia recently offered to hold military-to-military talks with Washington after covertly deploying troops, artillery units and tanks to Syria.

Many Western officials suspect the assets will be used to further prop up President Bashar al-Assad, despite claims they will be used to tackle IS militants.

"It's difficult to discern exactly what their most important priority is," Earnest said.

Later on Friday, a US official said that Russia had deployed four fighter jets to an airbase in Syria where it has been building up forces in recent weeks, alarming Washington.

The official, speaking on condition of anonymity, confirmed reports that four Russian jets had been seen on the ground at Latakia air base in western Syria, the heart of a Russian force build-up in the country.

Putin has provided vital support to Assad throughout a popular uprising against his rule and as the conflict has metastasized into a brutal civil war that has killed 240,000 people and displaced millions.

But Moscow has also sought to portray Assad's army as a bulwark against IS.

Washington and European states view Assad as a pariah who shoulders blame for driving Syria into chaos and allowing IS to thrive.

But with Western efforts to tackle IS floundering, and the moderate Syrian opposition losing ground to radical groups, the White House may hope to enlist Russia in the fight.

It was not immediately clear whether the US-Russia discussions on Syria would be held by the military or civilians, or at what level.

'Practical discussions with the Russians'

Military dialogue between Russia and the United States has been virtually suspended since 2014 in response to Moscow's annexation of the Crimea region of Ukraine.

Russia's support for eastern rebels in Ukraine had resulted in sanctions that have hollowed the Russian economy.

Western diplomats say Putin may now be trying to come in from the cold by exploiting the West's desire to end Syria's bloody conflict that has prompted a stream of refugees to Europe.

The Russian leader is expected to use his speech at the United Nations General Assembly later this month to push for a diplomatic solution to the conflict that allows Russia to retain its sole bulwark of influence in the Middle East.

The White House has so far rebuffed suggestions that Obama would meet Putin on the sidelines of the meeting.

Instead, the administration said it would use lower level talks to urge Russia to focus its actions in Syria on countering IS.

"We have made clear that Russia's military actions inside of Syria, if they are used to prop up the Assad regime, would be destabilising and counterproductive," Earnest said.

"That all being said, we have long indicated we could welcome constructive contributions from the Russians to the anti-ISIL coalition," he said, using an acronym for the Islamic State group.

"That is why we remain open to tactical, practical discussions with the Russians in order to further the goals of the counter-ISIL coalition and to ensure the safe conduct of the coalition operations."

UN envoy in Syria

Meanwhile, the UN's Syria envoy Staffan de Mistura on Thursday discussed his peace proposals with Syrian Foreign Minister Walid Muallem in Damascus

The meeting came as government helicopters attacked a rebel-held town near the flashpoint city of Daraa with barrel bombs, killing at least 21 civilians including women and children.

"We will continue the meetings," the UN's pointman on Syria said after their talks, declining to elaborate.

According to state news agency SANA, de Mistura and Muallem addressed the government's questions about the envoy's proposed 60-page peace plan.

The initiative, set to begin this month, would set up four working groups to address safety and protection, counter-terrorism, political and legal issues and reconstruction.

But de Mistura said the groups' work would be "for brainstorming and would not be binding," according to his office.

Syria's government had not wanted the committees' conclusions to be mandatory, a diplomat in Damascus said.

Muallem for his part said "fighting terrorism in Syria is the priority" and "the gateway to a political solution".

'Fighting terrorism demands first the end of the conflict'

De Mistura also met with internal opposition figure Hassan Abdel-Azim, who said he was ready to participate in the envoy's plan.

"But fighting terrorism demands first the end of the conflict between the regime and the opposition," Abdel-Azim said.

Al-Watan newspaper, close to the government, said Damascus and Moscow were "on the same page concerning the solution to the crisis" but that the UN had different priorities.

The Syrian and Russian leaders "have signalled that there is no political solution without defeating terrorism. It's the only way to put an end to the war in Syria," it wrote.

But it said de Mistura's plan "is aligned with the positions held by the 'opposition coalition,' America, Turkey and Saudi Arabia, who want the political solution to come before the fight against terrorism."

In an interview with Syrian state television, Muallem said Russia was ready to help Syria "fight terrorism," but denied Moscow was getting involved in "Syria's internal affairs".

US Secretary of State John Kerry said on Wednesday that Moscow had proposed opening a "military-to-military conversation" to ensure Russian forces do not come into conflict with the US-led coalition fighting IS.

Russia has reportedly moved artillery units and tanks to an airport in Assad's coastal stronghold in Latakia province, along with dozens of personnel and temporary housing for hundreds more.

'Stability at all costs'

Experts said Moscow's staunch backing for Assad combined with the arrival of thousands of Syrian asylum-seekers may push Europe to adopt a new approach towards the Syrian government.

"Indeed, after the migrant crisis, we heard several European voices pleading for a closer cooperation with Assad and (Russian President Vladimir) Putin," said Karim Bitar, head of research at the Institute for International and Strategic Relations.

"Clearly, the 'stability at all costs' narrative is rapidly gaining ground."

On Thursday, government airstrikes on Raqqa, IS's de facto capital in Syria, killed 18 people, including militants and civilians, the Syrian Observatory for Human Rights said.

On Tuesday, rebel rocket fire into government-held districts of Syria's northern city of Aleppo killed at least 38 people, among them 14 children.

And government air raids on Aleppo city in the last 24 hours also left 53 people dead, including 13 children, the monitor said.

The EU commissioner for humanitarian aid, Christos Stylianides, issued a statement on Thursday to "strongly condemn" attacks on civilian areas.

Barrel bombs dropped by government helicopters on Thursday killed 21 civilians, including two children and four women, in the southern town of Busra al-Sham near Daraa.

Rebel groups seized full control of Busra al-Sham, an ancient town with both Sunni and Shia Muslim residents, in March.

It was a major setback for forces loyal to Assad in the province where protesters first rose up against his rule in March 2011.

Assad's forces waged a bloody crackdown on the demonstrations, before people took up arms, leading to a full-blown war.

Since then, more than 240,000 people have been killed and millions have been forced to flee their homes.