US President Barack Obama has said a surface-to-air missile fired from a rebel-held area in east Ukraine brought down Malaysia Airlines flight MH17.
The plane crashed on Thursday, killing all 298 people on board.
Mr Obama called the attack "an outrage of unspeakable proportions", saying he would ensure "the truth is out".
The US and Russia clashed at the UN Security Council, with Russia's envoy putting the blame on Ukraine for its army's attacks on eastern areas.
Some 30 international investigators from the Organization for the Security and Co-operation in Europe (OSCE) reached the crash scene near the village of Grabovo on Friday.
However, Swiss Ambassador to the OSCE Thomas Greminger said the team did not get full access and were stopped by "local illegal armed groups".
At the scene: BBC's Daniel Sandford in Grabovo
The most noticeable thing is how far the wreckage has been spread across the wheat and sunflower fields. Small pieces of insulation, slivers of fuselage, large chunks of the tail.
And a terrible burnt tangle of bodies, aircraft seats and luggage. Guidebooks of Bali and Lombok have fallen near bags saying "I love Amsterdam". Some 24 hours after the crash, parts of the wreckage are still smouldering.
Each body that has been found so far is marked with a rudimentary stick with a white piece of cloth tied to it. The marker sticks are not just in the wreckage - they are dotted across the fields.
Nobody has secured the site. A few rebels in ad hoc uniforms are walking around with guns and there are a few firemen. As we drove away, we kept passing more pieces of fuselage several miles from the scene.
The Boeing 777 was flying from Amsterdam to Kuala Lumpur. It fell between Krasni Luch in Luhansk region and Shakhtarsk in the neighbouring region of Donetsk.
Latest figures released by Malaysia Airlines show the plane was carrying 189 Dutch nationals, 27 Australians, 44 Malaysians (including 15 crew), 12 Indonesians and nine Britons, along with a number of other nationalities.
'Unforeseen consequences'Confirming one American was among the plane's victims, Mr Obama said: "Evidence indicates that the plane was shot down by a surface-to-air missile that was launched from an area that is controlled by Russian-backed separatists inside of Ukraine."
OSCE monitors (right) reach the crash site, escorted by rebels He said in a briefing at the White House: "I think it's important for us to recognise that this outrageous event underscored that it is time for peace and security to be restored in Ukraine.
"Violence and conflict inevitably lead to unforeseen consequences."
Mr Obama called for an immediate ceasefire in eastern Ukraine. He demanded that full access be granted to investigators and that evidence should not be tampered with.
He said: "This was a global tragedy... The eyes of the world are on eastern Ukraine, and we are going to make sure that the truth is out."
World-renowned Aids researcher Joep Lange from the Netherlands is among those killed - Netherlands: 189
- Malaysia: 44 (including 15 crew)
- Australian: 27
- Indonesia: 12
- UK: 9
- Germany: 4
- Belgium: 4
- Philippines: 3
- Canada: 1
- New Zealand: 1
- US: 1
- Unverified: 3
MH17 crash brings Ukraine conflict 'to West's door'
Which conflict zones do planes fly over?
Mr Obama said it was up to Russia to stop the flow of heavy armaments and fighters into Ukraine.
Earlier at the UN, US envoy Samantha Power said the US could not rule out that Russians had helped the separatists fire the missile.
Ms Power added: "President Putin has committed on several occasions to working towards dialogue and peace, and every single time he has broken that commitment.
"Russia can end this war. Russia must end this war."
She said the UN had a "duty to everyone to determine why that jet fell out of the sky and stop at nothing to bring those responsible to justice".
US ambassador to the UN Samantha Power: ''Separatist forces backed by the Russian government continue to destabilise Ukraine''
The UK demanded that the UN go beyond its call for an inquiry and issue "a clear unequivocal condemnation of the actions of these armed groups" and insist that the rebels lay down their arms.
The UN's joint statement called for a "full, thorough and independent international investigation".
Russia's UN Ambassador Vitaly Churkin said:" Pressure should not be brought on this investigation, trying to prejudge its outcome with broad statements and insinuations that are unjustified."
He said the Ukrainian military was responsible for "punitive operations on civilian targets and infrastructure, with dozens of civilians killed", saying the attacks must be stopped as soon as possible.
Recordings purport to show pro-Russian separatists talking about the downing of a civilian plane
Senior vice-president of Malaysia Airlines Huib Gorter said the plane was sound
Mr Churkin also criticised the US, which he said had "pushed Ukraine to escalate the crisis and passed the blame on to Russia".
He also questioned why the Ukrainian aviation authorities had not closed the air space earlier.
'Act of terrorism'The OSCE says the separatists have pledged to provide "safe access" and co-operate with Ukrainian authorities.
Identities of the victims are becoming known, including WHO spokesman and former BBC man Glenn Thomas 
A vigil is held in Kuala Lumpur, Malaysia However, separatist leader Alexander Borodai rejected international calls for a truce, as fighting continues between the rebels and Ukrainian forces.
The rebels have accused the Ukrainian government of downing the airliner.
Ukraine called the disaster an "act of terrorism", blaming Russia, who it says has been aiding the rebels in the conflict and supplying them with advanced weapons.
Ukrainian authorities have released what they say are intercepted phone conversations that proved the plane was shot down by pro-Russian separatists.
MH17 is the second disaster suffered by Malaysia Airlines this year.
Flight MH370 disappeared en route from Malaysia to China in March and has still not been found.
The dead on MH17 include world-renowned Dutch researcher Joep Lange who was among a number of passengers en route to an international Aids conference in Australia.
Buk surface-to-air missile system
Also known as SA-11 Gadfly (or newer SA-17 Grizzly)
Russian-made, mobile, medium range system
Weapons: Four surface-to-air missiles
Missile speed (max): Mach 3
Target altitude (max): 22,000 metres (72,000ft)
Source: Global Security
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Airliner hit from rebel area - Obama
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