Washington is closely monitoring media reports claiming Russia has deployed its contingent to Syrian lands, White House spokesman Josh Earnest has said.
"We are aware of reports that Russia may have deployed military personnel and aircraft to Syria, and we are monitoring those reports quite closely," Earnest said.
“The US considers that the support for Syrian President Bashar Assad is 'destabilising and counterproductive'," he added.
It was not clear if Washington believes the allegations have been earlier rejected by the Kremlin.
The media outlets reported the Russian military deployment in Syria that claimed equipment has already been transferred to conduct the operations.
On Wednesday, Kremlin spokesman Dmitry Peskov rejecting media reports that surfaced about possible actual military participation of Russian Armed Forces in air strikes against ISIS, told Russia Today "You should not believe these media reports."
ISIS militant group currently controls large swaths of Iraq and Syria, taking advantage of the security vacuum in both countries.
"I saw reports about Russian planes on the Syrian territory. I also saw comments on this matter by Russia’s Defense Ministry and presidential administration that disavowed this information. There is nothing to add here," said Russian Foreign Ministry’s official spokesperson Maria Zakharova on Thursday.
Israeli newspaper "Yedioth Ahronoth" had first reported that Russia began actual military intervention in Syria, by acquiring a permanent air base, likely near Damascus, which will be used “to conduct airstrikes against ISIS and extremist formations in Syria.”
The Israeli Newspaper added that in the coming weeks, thousands of military Russian personnel will arrive in Syria, including advisers, trainers and field logistical support staff, as well as pilots.
“There is no doubt that the flight of Russian pilots over Syria to combat ISIS will change the current equations in the Middle East,” said Yedioth Ahronoth in its report.
“Russians do not harbor aggressive intentions toward Israel or other sovereign states in the region, and the aim of the military intervention is to fight ISIS and preserve Assad's rule,” said the Israeli newspaper.
Russia has been a key ally to Syrian President Bashar al Assad, along with Iran. The Syrian civil war has been on going for over four years, with a death toll that exceeds 240,000 according to the UK based Syrian Human Rights Observatory.