US surveillance drone that crashed after a Russian military jet likely broke it apart would be difficult to recover given the depth of water in Black Sea, says Washington.
The tragedy has exposed decades of safety failings in Greek railways and has put major pressure on the conservative government ahead of national elections.
More than 40,000 people protested nationwide, including an estimated 25,000 in capital, Athens, while a 24-hour strike forced the civil service, flights and ferries to shut down.
"Elements of planning and availability of resources" indicate that Russian authorities "may have committed torture as crimes against humanity", UN report says as conflict enters its 386th day.
Moscow accuses the West of directly participating in the war by supplying weapons to Ukraine after Slovakia became the second of Kiev's allies to provide MIG-29 fighter jets on the 387th day of fighting.