Russia has in recent weeks sent military hardware and personnel to Syria to back up the regime of Bashar al-Assad, a monitoring group reported on Sunday.
The Syrian Observatory for Human Rights said that Russian military planes carrying military equipment and hundreds of experts, as well as technicians, had arrived at the Hamim airbase near the coastal city of Latakia.
Russia also plans to construct a runway at the airbase so as to be able to handle large military aircraft, the Britian-based watchdog said without giving details.
Russian Foreign Minister Sergei Lavrov on Sunday said that his country would continue to support al-Assad's regime in what he called "its efforts to fight terrorism."
In recent months, the Syrian troops have suffered a series of military setbacks by rebels, mainly the ISIS terror organization.
The United States has expressed worries about media reports that Russia might be sending servicemen to Syria as part of a military buildup to support al-Assad's regime, who is locked in a conflict with a wide array of rebels fighting to oust him.
The US is spearheading an air campaign in Syria against ISIS, which rules vast territory of the country. However, it refuses to cooperate with Syrian forces.
Lavrov on Sunday criticized the West's approach to Syria's conflict.
"It's foolish to block the Syrian army from the fight with ISIS. The most efficient military force in the country is Syria's armed forces," he said in a TV interview on Sunday.
He added that Western leaders would say in private that ISIS is a much bigger threat than al-Assad, but refuse to say so in public. "They're afraid of losing face."
Washington has denounced Russia's military supplies to Syria, saying the support enables al-Assad to avoid negotiating for a political solution to the conflict, now in its fifth year.
Syria's crisis has claimed more than 250,000 lives, according to the United Nations.