A Russian military buildup in Syria has left Western officials scrambling to clarify Moscow’s intent in the war ravaged country.

On Tuesday, US Secretary of State John Kerry made his third phone call in 10 days to his Russian counterpart, demanding answers for Moscow’s continued support of Syrian President Bashar Assad.

US State Department John Kirby confirmed Kerry’s phone call with Russian Foreign Minister Sergey Lavrov. During the conversation, Kerry “made it clear that Russia’s continued support for President Assad risks exacerbating and extending the conflict, and undermining our shared goal of fighting extremism if we do not also remain focused on finding a solution to the conflict in Syria via a genuine political transition.”

Two US officials revealed earlier this week that Russia has positioned more than half a dozen T-90 tanks at an airfield near Latakia, a stronghold of Assad. Russia has also strategically placed artillery positions in a defensive maneuver meant to protect Russian troops stationed nearby.

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The officials believe Moscow’s latest actions in Syria suggest the desire and intent to expand a Russian military buildup with continued and increased support for Assad.

Russian President Vladimir Putin defended his country’s continued assistance to Assad, arguing that it is impossible to defeat ISIS without cooperating with Syria. “This would not be the first situation in which President Putin’s motivations are rather hard to discern,” White House spokesman Josh Earnest told reporters.

However, Syria’s ambassador to Russia, Riad Haddad, called reports of Russian troop presence in Syria “a lie.”

“We have been cooperating with Russia for 30-40 years in various areas, including the military sphere. Yes, we receive arms, military equipment, all this is done in line with agreement sealed between our countries,” Haddad was quoted as saying by ITAR-TASS news agency.

“But the talk of your (Russian) troop presence on the Syrian territory is a lie spread by Western countries, the United States,” he added.