The bodies of the 97 people who died when a plane carrying Polish President Lech Kaczynski crashed in thick fog in west Russia on Saturday morning are being flown to Moscow.
Kaczynski's wife and a delegation of senior Polish officials, including the chief of the General Staff and the head of the national bank, were also on board.
A highly-placed police source earlier told RIA Novosti that Kaczynski's body may have already been found, but that "additional tests, including DNA," would be needed to identify many of the bodies.
The Polish president was on his way to pay tribute to the some 20,000 Polish officers who were executed by Soviet secret police in Katyn forest and other locations in 1940. The officers had been taken prisoner by Soviet forces when the USSR invaded Poland during the early days of World War Two.
Cause of Crash
Russian television pictures from the crash site showed a scene of utter devastation, with emergencies ministry workers attempting to extinguish the flaming, scattered remains of the aircraft.
"The causes of the tragedy should be established in the shortest time possible within the framework of the government commission," Putin said after arriving at the scene of the disaster.
Shoigu said that a recording of conversations between the pilot and air traffic controllers showed that the pilot had ignored advice not to attempt to land at the airport.
Air traffic controllers suggested that he turn around and head for Minsk, the capital of neighboring Belarus, as the military aerodrome lacked the necessary navigational equipment to receive planes in heavy fog.
Lieutenant General Alexander Alyoshin, deputy head of Russia's air force, said that the pilot had also ignored orders from air traffic controllers "to return to horizontal flight."
"When the crew did not follow the instructions, they were ordered several times to land at another airport." he said.
Russian President Dmitry Medvedev has pledged a "thorough" investigation into the crash.
Poland will observe seven days of national mourning, parliamentary speaker and acting president, Bronislaw Komorowski, said.
"We must all come together in the face of this massive national tragedy," he added.
"Today, there is no left wing and no right wing - there is no difference. We all feel for the families of the dead," he said.
Relatives of the victims of the crash left for Russia shortly after the news of the tragedy broke and began arriving in Moscow at around 8:00 p.m. local time (16:00 GMT).
Medvedev and Putin have called the crash "a terrible tragedy."
Putin and Polish Prime Minster Donald Tusk have laid flowers together at the scene of the crash.
Russia will observe a day of national mourning on April 12. In Moscow, people have been bringing flowers to the Polish embassy.
"People have died - people have perished on our territory. I think that many people who couldn't come here empathize with the families of the dead, who have left this world for another place," Muscovite Stanislav Rodzishevsky told RIA Novosti.
UN Secretary General Ban Ki-moon said he was shocked to hear the news of the crash.
"On behalf of the United Nations, the Secretary-General expresses his deep and heartfelt condolences to the people and government of Poland and to the families of those who perished," Ban's spokesman said.
U.S. President Barack Obama said his "thoughts and prayers are with the Kaczynski family, the loved ones of those killed in this tragic plane crash, and the Polish nation."
"Today's loss is devastating to Poland, to the United States, and to the world. President Kaczynski was a distinguished statesman who played a key role in the Solidarity movement, and he was widely admired in the United States as a leader dedicated to advancing freedom and human dignity," he went on.
British Prime Minister Gordon Brown said Kaczynski would be "mourned across the world."
"President Kaczynski was one of the defining actors in Poland's modern political history," Brown said. "From his role in the Solidarity movement to his long and distinguished career in public service."
"All of Germany is mourning and standing with you," German Chancellor Angela Merkel wrote to Polish Prime Minister Donald Tusk.
French President Nicolas Sarkozy said Kaczynski's death was a tragedy for Poland.
"France loses a friend who was profoundly attached to developing relations between our two countries," the French leader said.
EU foreign policy chief Catherine Ashton also expressed her condolences.
MOSCOW, April 10 (RIA Novosti)