A US fighter jet has shot down a pro-Syrian regime drone, marking at least the fourth time in a month that American forces have directly clashed with Bashar al-Assad’s troops and their allies and raising fears of a broader escalation in Syria.

US officials said the armed Iranian-made drone was shot down on Tuesday as it neared the al-Tanf base in southern Syria where American and British special forces train Syrian rebels for the fight against Islamic State (Isil).

The drone was blown up two days after the US destroyed a Syrian regime warplane for the first time, prompting Russia to threaten any coalition aircraft that entered areas where its air forces are flying.

The Russian threat prompted Australia to suspend its bombing missions in Syria as part of the anti-Isil coalition, and raised fears that the US may be edging towards a full-on confrontation with Iran and Russia.

“This is a dangerous escalation,” said Christopher Murphy, a Democrat US senator. “We have to understand what we’re getting involved in. You’re not just fighting Bashar al-Assad. If you’re going to ramp up militarily against Assad you’re also going up against Iran and Russia.”

Meanwhile, an armed Russian warplane flew within five feet of a US reconnaissance aircraft over the Baltic Sea on Monday. US officials said the armed Su-27 buzzed past the American aircraft in a way that was “provocative” and “unsafe”.