UPDATE: 4:50 a.m. EDT -- British Defense Secretary Michael Fallon said Tuesday that there have been no discussions with Russia about any military action in Syria, Reuters reported.

"We have not discussed military operations against ISIL (Islamic State group) in Syria with the Russians," Fallon reportedly told the Royal United Services Institute, a think tank. "The Russian action in the last few weeks of putting ships and aircraft into the region obviously further complicates an immensely complex situation." 

Meanwhile, Iran's Deputy Foreign Minister Hossein Amir Abdollahian said Moscow and Tehran would work together to help end the crisis in Syria, Reuters reported, citing RIA news agency. 

"Tehran and Moscow intend to use all possibilities and potential to help Syria come out of this crisis," he reportedly said, at a news conference in Moscow, adding that his country had no troops or military advisers in Syria or Yemen.

Original story:

Russia has increased the number of combat aircraft at an air base in the western Syrian province of Latakia, U.S. officials said Monday. Twenty-eight fighter and bomber aircraft have been deployed in the war-torn nation, giving rise to concerns over Moscow’s moves to support Syrian President Bashar Assad.

Twelve Su-24 Fencer and 12 Su-25 Frogfoot ground-attack planes have been deployed in the base, where only four Flanker air-to-air fighters were present until the weekend, the New York Times reported, citing a senior U.S. official. In addition to the combat aircraft that were deployed over the weekend, a total of 15 Russian Hip transport and Hind attack helicopters are also stationed at the base, the official reportedly said.

“The equipment and personnel just keep flowing in,” the official, who did not want to be named, told the Times. “They were very busy over the weekend.”

Another official told Agence France-Presse (AFP) that Russia was also operating drones over Syria, but did not give out any other details. According to Reuters, a drone surveillance mission is underway in Syria, leading to a risk of U.S.-led coalition planes and Russian aircraft operating within the Middle Eastern country’s limited airspace.

"They are not going to sit around and defend the airfield or maybe even the province of Latakia,” analyst Jeffrey White of the Washington Institute for Near East Policy said, according to AFP. "This kind of aircraft suggests that the Russians intend to exert their combat power outside of Latakia in an offensive role.”

U.S. Secretary of State John Kerry said over the weekend that the country welcomed Russia's involvement in fighting Islamic State group militants in Syria. But, according to reports, Russia’s intentions remain unclear as Washington opposes Moscow's support for Assad.

Meanwhile, some experts speculate that Moscow might eventually target opposition fighters that the U.S. supports in Syria, viewing them as threats to Assad.

John Kirby, a spokesman for the U.S. State Department, also expressed concerns over Russia’s move, saying that it’s unclear whether Moscow aims to battle ISIS in Syria or "prop up the Assad regime."