Russian and US fighter jets found themselves in a tense standoff in Syria, after Russia bombed opposition forces allied with the US.

The incident occurred last week, while a group of about 200 rebels was fighting against ISIS near the border with Jordan. Russian SU-34 jets appeared during the course of the fighting and struck the rebels. In response, the Pentagon ordered its own F/A-18 fighter jets to the site.

The Russians cleared out when the newcomers arrived, but then attacked again when the F/A-18s left to refuel. The second wave of attacks killed a number of paramedics who were treating victims from the first strike.

An official statement from the Pentagon noted that "Russia's continued strikes at At-Tanf, even after US attempts to inform Russian forces through proper channels of ongoing coalition air support to the counter-ISIL forces, created safety concerns for US and coalition forces," using another name for ISIS.

"It’s an egregious act that must be explained," a U.S. official told the Los Angeles Times. "The Russian government either doesn't have control of its own forces or it was a deliberate provocative act. Either way, we’re looking for answers."

For its part, Russia denies that it intentionally hit US-backed rebels and says that the US is at fault for not warning Moscow about the rebels' location.

There are numerous competing factions in Syria. Russia supports dictator Bashar al-Assad and claims to be using its airstrikes to attack ISIS. Rebels, which oppose both Assad and ISIS, claim that many of the airstrikes are really used against opposition forces.