Russia has warned the US military twice in the last week that its forces, along with Syrian regime units, are prepared to attack in an area where dozens of US troops are located, according to several US defense officials. Russia claims that there are militants in the area protected by US troops. Moscow’s declaration has sharply raised US commanders’ concerns that American forces would be at risk if a Russian attack goes forward, CNN has learned.

And it has sparked US warnings to Moscow not to challenge the US military presence. Several US defense officials have told CNN that concerns center on a US-led anti-ISIS coalition base at At Tanf. US troops help monitor a 55-kilometer (34-mile) exclusion zone around At Tanf. Given its location near the borders of Syria, Jordan and Iraq, the At Tanf garrison is seen as a key strategic location as the US, Iran and Russia compete for influence in the region. READ MORE