A Russian airliner carrying up to 78 passengers and crew on a flight from Israel exploded and plunged into the Black Sea on Thursday in unexplained circumstances.
Ukraine quickly dismissed American suggestions that the plane might have been hit by an accidental missile strike from the Ukrainian military.
The mid-air explosion of the Sibir airlines Tupolev-154 jet, on a scheduled flight from Tel Aviv to Novosibirsk in Siberia, inevitably triggered fears of sabotage following the September 11 airliner attacks in the United States.
But late on Thursday, President Vladimir Putin called on the media not to "sensationalise" the accident, backing down from earlier comments that the crash might be a "terrorist act".
"Specialists will be able to come to final conclusions about the causes of the accident only after a thorough investigation and laboratory tests of the plane's wreckage," Putin told a news conference alongside British Prime Minister Tony Blair, in Moscow to discuss the international crisis.
A US official in Washington said: "It could well have been a training accident, it could have been the Ukrainian military conducting a live-fire test. It could be a tragic accident."
The official, who did not wish to be identified, noted that Ukraine had been test-firing live surface-to-air missiles from the Crimea at the time.
But Ukrainian military spokesman Konstantin Khivrenko said: "Neither the direction nor the range (of the missiles) correspond to the practical or theoretical point at which the plane exploded. So the Ukrainian military has no involvement, either practical or theoretical, in this accident."
And Russian security sources, quoted by Interfax news agency, said the Ukrainian exercises had been taking place more than 320 km (200 miles) from where the plane came down.
Putin confirmed the missiles used by the Ukrainian forces did not have the required range.
"The weapons used by the army in the exercises were by their technical characteristics unable to reach the corridor through which the TU-154 was travelling," he said.
The plane went down 190 km (110 miles) south of the Black Sea port of Novorossiisk in water about 1,000 metres (3,300 feet) deep, an Emergencies Ministry official said. Russia sent a rescue ship and plane to the scene.
Russian television showed pictures of fragments of the airliner floating in the sea as rescuers searched for bodies.
Official accounts of the number of passengers and crew on board varied between 74 and 78. An Emergencies Ministry official in Novosibirsk said the crew were all Russian but that most of the 65 or so passengers were Israeli citizens.
A Reuters reporter who watched the Ukrainian exercises said surface-to-air missiles were being fired from the east side of the Crimean peninsula at 20 or more airborne drone targets at around the time the plane came down.
PILOT SAW BLAST
In the Armenian capital Yerevan, Garik Ovanesyan, the flight director of Armenian Airlines, said the pilot of a plane flying near the Sibir jet had seen flames coming from it.
"The commander...contacted ground control to find out if there were any (military) exercises. Then there was an explosion and fragments falling to the sea," he said, citing the pilot.
Nikolai Patrushev, the head of Russia's domestic FSB security service, passed Armenian reports of the mid-air explosion to Putin during a crisis meeting in the Kremlin.
Putin spoke by telephone to Israeli Prime Minister Ariel Sharon following the crash, the Kremlin press service said.
Itar-Tass news agency quoted the local Emergencies Ministry as saying 11 bodies - nine women and two men - had been recovered from the crash site.
Vladimir Rushailo, head of Russia's Security Council, is to head a commission to look into the causes of the crash. NTV television said Rushailo arrived in the Black Sea resort of Sochi late on Thursday along with FSB experts.
The disaster, whatever its cause, provided a grim background to a meeting which took place between the Kremlin leader and Blair, who was set to leave Moscow early on Friday.
The September 11 attacks have led Washington to declare a "war on terrorism" and prepare military action against bases of Saudi-born militant Osama bin Laden in Afghanistan.
Interfax quoted the FSB as saying: "Taking into account the latest events in the world, the theory of a terrorist act is being investigated first of all."
A Sibir airlines official in Novosibirsk said the plane would not normally have passed over the Black Sea and appeared to have been off course.
"Why should they have been given such an air corridor?" she asked.