KIEV - Analysis of the black box flight recorders from the downed Malaysia Airlines Flight MH17 shows that it was destroyed by shrapnel from a rocket attack which caused "massive explosive decompression".
The attack was now being investigated as a war crime, the United Nations said.
Kiev and the West accuse pro-Russian rebels of shooting down the plane, and the European Union is drafting tougher sanctions against Russia.
Sanctions targeting entire economic sectors are being considered, including an arms embargo, and the 28-member bloc is expected to unveil more names of individuals and entities sanctioned.
Moscow, however, has blasted EU's move as "irresponsible". It says the Ukrainian government is responsible for the attack on the plane, which killed all 298 people on board.
The spokesman for Ukraine's Security Council, Andriy Lysenko, told a news conference here that experts analysing the recorders from the plane had concluded that a rocket attack brought the plane down in rebel-held territory in eastern Ukraine on July 17.
Britain has been tasked with downloading the data from the two black boxes recovered from the site.
A Dutch-led investigation into the crash, however, has made little headway due to the intensifying fighting in the insurgent-held zone.
The fighting blocked a new attempt by Dutch and Australian police to reach the crash site.
The unarmed international mission has been forced to turn around as heavy bombardment rocked towns close to the site.
The Red Cross has said Ukraine is now in civil war - a classification that would make parties in the conflict liable to prosecution for war crimes.
UN High Commissioner for Human Rights Navi Pillay condemned the "horrendous shooting down" of the jet and demanded a "thorough, effective, independent and impartial investigation".
"This violation of international law, given the prevailing circumstances, may amount to a war crime," she said.
Pillay warned that both sides were "employing heavy weaponry in built-up areas, including artillery, tanks, rockets and missiles".
"Both sides must take great care to prevent more civilians from being killed or injured," the high commissioner said.
The fighting continued overnight, as the Ukrainian army appeared to be intensifying its offensive to wrest control of the industrial east and cut off the rebels from the Russian border.