Russia’s decision to deploy its most hi-tech air defense system to its base in Syria is raising “significant concerns” for the United States military, an American official said Wednesday, according to AFP.

The statement was made after Moscow said it is sending S-400 anti-aircraft missiles to Latakia in northwestern Syria, in a move that comes after Turkey shot down a Russian fighter jet in the increasingly crowded air space along the border on Tuesday.

The S-400 missiles have a range of about 400 kilometers (250 miles) — meaning they could reach deep into Turkey or pose a potential threat to U.S.-led coalition planes — adding yet another dangerous element to an already volatile mix of competing military interests in Syria.

“It’s a capable weapons system that poses a significant threat to anyone,” an American official speaking on condition of anonymity told AFP. “There are significant concerns related to air operations in Syria.”

The United States has since August 2014 led a coalition that has flown more than 8,000 bombing runs against Islamic State (ISIS) group targets in Syria and Iraq.

Russia, too, is dropping bombs in Syria but these are mainly in different parts of the country from where U.S. and coalition planes are flying. The West says Russia is propping up the regime of President Bashar Al-Assad, and not focusing on ISIS jihadists.

Though Russia and the U.S.-led coalition have agreed on a set of guidelines aimed at ensuring pilots stay out of each other’s way, the prospect of batteries of Russian anti-aircraft missiles arriving in Syria is nonetheless raising eyebrows in the Pentagon.

Another American official, also speaking anonymously, said, however, that the S-400s “shouldn’t” affect coalition flights.

“We are not going to interfere with (the Russians’) operations and they are not going to interfere with ours. There’s no reason for us to be targeting each other,” the official said.

He also noted that Russia in the past week has delivered more than 30 T-90 and T-72 tanks to Latakia.

It was not clear if these were for use by the Russian military or will be provided to forces loyal to Assad.

On Tuesday, Turkey shot down a Russian aircraft along the Syrian border, and rebels killed one of the pilots as he parachuted down after ejecting from the plane. A second pilot was rescued by Russian and Syrian special forces.

A Russian rescue helicopter was also destroyed by rebels, who apparently used an American-made TOW missile.

The prospect that Syrian rebels used U.S. weaponry to kill a Russian further raises concerns that the Syria conflict could devolve into a proxy war.

AFP contributed to this report.