The United States has reportedly intensified efforts to block Russian flights to Syria to prevent Moscow from what it describes as a military buildup in the war-torn country.

“We have encouraged our allies and partners to ask tough questions of Russia’s increased military deployments to Syria,” The New York Times quoted John Kirby, the US State Department spokesman, as saying on Tuesday.

The Obama administration is accusing Russia of making preparations to deploy advisers and military personnel to an airfield south of Latakia and might also bring in aircraft and fly airstrikes from there.

The intelligence reports said last week that those preparations included the delivery of prefabricated housing for as many as 1,000 personnel and a portable air traffic control station to the airfield, according to the Times.

On Saturday, US Secretary of State John Kerry called his Russian counterpart Sergei Lavrov and warned that Washington is deeply concerned of reports of "an imminent enhanced Russian military buildup" in Syria.

"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIL coalition operating in Syria," the State Department said.

On Monday, a spokeswoman for Lavrov expressed surprise over the warning and said that Russia had long provided military aid to the government of Bashar al-Assad in its fight against the Daesh Takfiri terrorists.

Syria has been gripped by deadly unrest since March 2011. According to reports, the United States and its regional allies - especially Qatar, Saudi Arabia, and Turkey - are supporting the militants operating inside the country.

According to the United Nations, more than 230,000 people have been killed and millions displaced due to the turmoil that has gripped Syria for over four years.

The Daesh terrorists, who were initially trained by the CIA in Jordan in 2012 to destabilize the Syrian government, now control large parts of Iraq and Syria.

Since late September 2014, the US and some of its Arab allies have been carrying out airstrikes against ISIL inside Syria without any authorization from Damascus or a UN mandate.