Reports indicate Russia has increased its military presence in the war-torn country of Syria within the last week.

According to The New York Times, Russia has sent more aircraft to its air base near Lattakia. A U.S. official told The Times that Russia deployed 28 fighter planes, including 12 Su-24 Fencer and 12 Su-25 Frogfoot ground-attack planes.

CNN has also reported Russia has begun to launch surveillance drones into the area, and has deployed several helicopters, tanks and close to 500 marines.

Russia's military increase in Syria has the U.S. government concerned that they plan to take offensive action in support of President Bashar al-Assad's regime. The U.S. is also worried about the risk of clashing with Russian forces, as the Americans lead the Western coalition in combating the terror group ISIS.

"We are keenly aware of what is happening there," Pentagon representative Capt. Jeff Davis told CNN. "We continue to believe that anything that's done in support of the Assad regime, particularly militarily, is counter-productive and risks worsening an already bad situation."

U.S. Defense Secretary Ashton Carter contacted Russian defense minister Sergei K. Shoigu to help ensure the two countries would take care to avoid any accidental conflicts as they operate in Syrian airspace.

In the past, the U.S. has supported Syrian rebels in fighting off ISIS militants. Russia, however, has condemned the Syrian resistance in the aftermath of an alleged attack on the Russian Embassy in Damascus.

"We expect a clear position in regard of this terrorist act from all members of the international community, including regional parties," the Russian Foreign Ministry said in a statement on Monday. "This requires not only words, but also concrete actions."

Secretary of State John Kerry said the U.S. would support Russia if its intention is to use military action to combat ISIS, but not if its goal is to empower the Assad regime.