A Russian fighter jet was shot down along the border between Turkey and Syria on Tuesday.
Russia said the plane was shot down by weapons on the ground.
But Turkey said its jets shot down the Russian plane when it wrongly entered Turkish airspace. The Russian plane ignored repeated warnings to leave, Turkish officials said.
Russian military officials said the Russian jet stayed within Syria as allowed.
The two pilots ejected, Russia said. Video showed two pilots parachuting from the sky. The pilots' location is unknown.
A U.S. State Department spokesman says they cannot confirm the reports. They said they are watching closely.
The latest incident comes after four and a half years of civil war in Syria. Border country Turkey has made several warnings about aircraft from other countries entering its airspace without permission.
Russia joined the conflict in Syria in September. It fights on the side of Syrian President Bashar al-Assad. Russia says it is fighting Islamic terrorists to help Assad return order in Syria. Fighters in the civil war against Assad say Russia is targeting them.
In October, Russia agreed to keep a safe distance and communicate with other governments to prevent mid-air disasters.