U.S Defense Secretary Lloyd Austin vowed Wednesday that US aircraft will fly "wherever international law allows" and warned Russia to operate with caution after one of its jets allegedly downed an American drone.

Austin made the statement shortly after speaking by phone with Russian counterpart Sergei Shoigu about Tuesday's incident over the Black Sea, when two Russian fighters allegedly harassed an unmanned US surveillance drone and damaged its propeller, forcing its ditching in the waters below.

The United States labeled the incident "reckless" and "unprofessional" while Moscow denied it was to blame and instead accused Washington of conducting "hostile" flights in the region.

"The United States will continue to fly and to operate wherever international law allows," Austin told reporters just following the call with Shoigu.

"And it is incumbent upon Russia to operate its military aircraft in a safe and professional manner," he said.

US Joint Chiefs Chairman Mark Milley said the Pentagon is still analyzing video and data from the drone to see exactly what happened.

"Was it intentional or not? - don't know yet," he told reporters.

"We know that the intercept was intentional. We know that the aggressive behavior was intentional, we also know it was very unprofessional and very unsafe," Milley said.

"The actual contact of the fixed-wing Russian fighter with our UAV, the physical contact with those two, not sure yet."

Austin expressed appreciation for the call, after more than a year of the Ukraine war during which direct contact between top US and Russian defense officials has been exceedingly rare.

"We take any potential for escalation very seriously and that's why I believe it's important to keep the lines of communication open," he said.

"I think it's really key that we're able to pick up the phone and engage each other. And I think that will help to prevent miscalculation going forward."