A passenger Boeing plane en route from Grozny to Moscow has sent a distress signal over the Caspian Sea, a source in aviation services told TASS Friday.

"A Boeing en route from Grozny to Moscow has sent a distress signal over the Caspian Sea and descended to 3,000 meters," the source said.

The plane took off from Grozny at about 15:00 Moscow time. The cause of distress has not been disclosed yet.