Story highlights Russian military moves inside Syria continued to escalate Friday with the arrival of fighter jets

Russia's defense minister said the recent Russian military buildup in Syria is "defensive in nature"

Washington (CNN) Defense Secretary Ashton Carter spoke with his Russian counterpart Friday as Russian military moves inside Syria continued to escalate with the arrival of fighter jets.

In a "constructive conversation," both Carter and Russian Defense Minister Sergei Shoygu agreed to "further discuss mechanisms for deconfliction in Syria," Pentagon Press Secretary Peter Cook said in a statement, as the United States and its allies continue military operations against ISIS inside Syria.

Shoygu told Carter that the recent Russian military buildup in Syria is "defensive in nature" and aimed at supporting Russian obligations to the regime of Syrian President Bashar al-Assad, a senior U.S. Defense official told reporters.

The U.S. policy is that Assad, a close ally of Moscow, must leave office in order to resolve the years-long civil war, and U.S. officials have warily watched Russia's military support of the Syrian regime.

However, with U.S. aircraft also operating in the country as America roots out ISIS, it is important that the two powers coordinate their military presence to avoid accidental encounters.

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