Ukraine has denied that any of its forces were responsible for firing a missile which may have caused a Russian airliner to explode.
US officials believe a surface-to-air missile fired from the Crimea area of Ukraine appears to have accidentally brought down the jet.
A Ukrainian spokesman said that their rockets' self-destruct mechanisms would have prevented the disaster.
The plane was flying from Tel Aviv to the Siberia city of Novosibirsk. It crashed into the Black Sea with 78 passengers and crew on board. All 66 passengers were Israelis.
The jetliner was seen to explode at 36,000 feet. President Vladimir Putin initially told a meeting of visiting European justice ministers that terrorism could be the cause.
Deputy Transport Minister Karl Ruppel said a crew of an Armenian An-24 airliner in the area had informed Russian air traffic controllers in Rostov-on-Don in southern Russia that they saw an explosion aboard a plane flying nearby.
The Armenian plane was flying on a regularly scheduled flight over the Black Sea from the Ukrainian Crimean city of Simferopol to the Armenian capital Yerevan.
Mr Ruppel could not immediately say what the cause of the explosion was. He was confirming Russian media reports that a blast had occurred on the Russian plane.
The plane, a Tupolev 154, went down in pieces 114 miles off the Russian coastal city of Adler, located on the Georgian border. It belonged to Sibir Airlines, which is based in Novosibirsk and had been chartered. The Emergency Situations Ministry said there were at least 66 passengers and 11 crew members aboard.