A US defense official says a Russian fighter jet flew dangerously close to an American warplane over eastern Syria.

The official said the two jets were as close as half a mile from each other. He said it was dark when the encounter occurred and both planes were flying without lights.

US officers asked Russia about the incident, and Russian officials said "the pilot didn't see" the American plane. US officials say the rate of close calls has increased over the past six weeks. They've accused Russian jets of intentionally flying close to coalition planes.

Syria, Russia, the US and Turkey all operate in Syrian air space.