US President Barack Obama described Thursday's crash, from which there were no survivors, as "a global tragedy."
Circumstantial evidence indicated Thursday's flight from Amsterdam to Kuala Lumpur was shot down over eastern Ukraine from 33,000ft by a surface-to-air missile fired from near the mining city of Torez, which lies in territory controlled by rebels seeking closer ties to Moscow.
Mr Obama stressed it was not the first time the rebels had shot down planes in the region, adding that a "steady flow of support from Russia" had included heavy weapons and anti-aircraft weapons.
He called for an immediate ceasefire to allow a full investigation. But last night international monitors from the Organisation for Security and Cooperation in Europe were being denied full access to the crash site by separatists, who apparently fired shots into the air. The whereabouts of the black box data recorder is also unclear, amid unconfirmed reports it may already be in Moscow.
The development raises fears vital evidence has been removed.
Messages on pro-Russian separatist websites immediately after the crash boasted of having shot down what the rebels claimed was a Ukrainian military transport plane. They have since been deleted.
Infants were among the 80 children who died, and toys were among the personal possessions scattered around the crash site.
Also among the victims were more than 100 delegates travelling to an international conference on Aids in Melbourne, Australia. They included Joep Lange, a world-renowned researcher, communications chief Jacqueline van Tongeren and British World Health Organisation worker Glenn Thomas.
But the Netherlands bore the brunt of the tragedy, with 154 of its citizens were killed.
The impact of the crash near the settlement of Grabovo, near the Ukraine border with Russia, scattered bodies over miles of rebel-held territory. As local men searched for victims, makeshift white flags were put down to mark where the bodies lay in corn fields and among the debris. UK police officers headed to the scene to attempt to repatriate the remains of the British victims.
One pensioner described how the body of a woman smashed though her roof. "There was a howling noise and everything started to rattle," explained 65-year-old Irina Tipunova. "Then objects started falling out of the sky. Then I heard a roar and she landed in the kitchen."
The British victims named so far include two Newcastle United fans, two undergraduates and a helicopter rescue pilot.
David Cameron, who chaired an emergency meeting of senior ministers and officials in Whitehall, described the crash as an "absolutely appalling, shocking, horrific incident" that "cannot be allowed to stand"; he insisted those responsible had to be brought to account.
The Prime Minister, who spoke to several world leaders including Mark Rutte, his Dutch counterpart, was due last night to talk to Mr Obama. Attempts were also being made to contact Russian President Vladimir Putin. A House of Commons statement is expected on Monday.
Sir Malcolm Rifkind, the former Foreign Secretary, said sanctions against Moscow should be dramatically toughened if it is established that Russian-supplied weaponry brought down the plane. But No 10 made clear the priority was to get the facts and to bring the victims home. Following a request from Mr Rutte, a British police team will assist the Dutch in the recovery and repatriation of the bodies.
In New York, the United Nations Security Council called for a "full, thorough and independent international investigation" into the crash.
Arseniy Yatsenyuk, Ukraine's Prime Minister, blamed the separatists as he demanded all nations support Kiev "to bring to justice all these b******* who committed this international crime".
Mr Putin placed the responsibility for the tragedy on Ukraine, saying it was responsible for the unrest in its Russian-speaking eastern regions.