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.
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.”
Confirmed death toll so far
World-renowned Aids researcher Joep Lange from the Netherlands is among those killed
Malaysia: 44 (including 15 crew)
New Zealand: 1
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”.