WASHINGTON — Long-running tensions between the United States and Russia erupted publicly on Monday as Moscow condemned the American military’s downing of a Syrian warplane and threatened to target aircraft flown by the United States and its allies west of the Euphrates.

The Russians also said they had suspended their use of a hotline that the American and Russian militaries used to avoid collisions of their aircraft in Syrian airspace.

The episode was the first time the United States downed a Syrian plane since the civil war began there in 2011 and came after the SU-22 jet dropped bombs on Sunday near American-backed fighters combating the Islamic State. It followed another major American military action against the Syrian government: a cruise missile strike to punish a nerve gas attack that killed civilians in April.

The latest escalation comes as competing forces converge on ungoverned swaths of Syria amid the country’s six-year civil war. Syrian forces and Iranian-backed militias that support them are extending their reach east closer to American-backed fighters, including forces that the Pentagon hopes will pursue the militants into the Euphrates River valley after they take the Islamic State’s self-declared capital of Raqqa. The collision of the disparate forces has, in effect, created a war within a war.