A team of international observers was given only limited access to the Malaysian airliner crash site in eastern Ukraine by separatist fighters guarding the area.
Ukraine has accused pro-Russian rebels of trying to destroy evidence of "international crimes" at the crash site. Kiev said the rebels were preventing international representatives and its own experts from starting their investigation.
It is believed flight MH17 crashed after being hit by a surface-to-air missile fired from a rebel-held area in east Ukraine on Thursday.
All 298 people on board died. The passengers included 80 children.
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, the BBC reports.
A Malaysian disaster response team, including two air accident investigators, was due in the Ukraine capital Kiev, after the country's leader appealed to Russia's President Vladimir Putin to help them gain access to the crash site.
The Malaysian Prime Minister Najib Razak says he has spoken to Mr Putin by phone to stress the need for an objective, unfettered probe into the crash, amid concerns the site is vulnerable to tampering.
OSCE team forced to leave
About 20 observers from the Organisation for Security and Co-operation in Europe (OSCE) reached the main crash site near the village of Grabovo on Friday but were stopped by what a spokesperson said were heavily armed men.
OSCE spokesperson Michael Bociurkiw said there didn't seem to be anyone really in control.
The men, apparently pro-Russia soldiers, loosely guarded the area but couldn't answer the monitors' questions, he said.
Mr Bociurkiw said the group only stayed about 75 minutes and examined about 200 metres at the scene before being forced to leave.
Pieces of the airplane and bodies are spread over several kilometres.
A member of the team said one visibly intoxicated guard fired his rifle into the air when an observer walked out of the prescribed area.
However, Mr Bociurkiw said the team was able to establish that the site remains intact.
"There's no apparent tampering of the debris. You have some fairly large pieces lying there in the fields.
"But it's astonishing to go there and see the scene with no recovery going on, with these bodies starting to decompose."
Mr Bociurkiw said the crash scene extends for about 6km.
Russia urged to stop supporting rebels
Meanwhile, international pressure is growing on Russia to stop supporting separatists in Ukraine after the downing of the Malaysian passenger plane MH17.
The United States says it now has evidence to show the airliner was hit by a surface to air missile, launched from the eastern Ukraine area controlled by pro-Moscow separatists.
Mr Putin is promising full co-operation with the investigation into the tragedy but continues to point the finger at Ukraine.
The United Nations Security Council, which has been holding an emergency meeting on the disaster, said there must be a full independent investigation.
The UN said 80 of the 298 people aboard Flight MH17 were children. The passengers were from 11 different countries.
Satellite images have been released showing a plume of smoke left by the ground-to-air missile that brought the airliner down.
United States analysts have been sifting through fragments of intelligence to try to pin down who fired the missile.
The US Ambassador to the United Nations, Samantha Power, has said the airliner was likely hit by an SA-11 surface-to-air missile operated from a separatist-held location.
Obama criticises Russia
Meanwhile, United States President Barack Obama is demanding Russia stop supporting separatists in Ukraine after the downing of the airliner.
Mr Obama has raised the prospect of more sanctions on Moscow.
Describing the deaths of almost 300 people an outrage of unspeakable proportions, he stopped short of directly blaming Russia for the incident. But he has called for a rapid and credible investigation, saying "we don't have time for games".
He urged an immediate ceasefire in the region, where fighting continues between the separatists and Ukraine forces.
Russia's United Nations ambassador, Vitaly Churkin, said it was too early to lay blame for the disaster, saying the outcome should not be predetermined by insinuations and false accusations.
Mr Churkin is also questioning why Ukranian airspace controllers allowed the plane to fly over a conflict zone in which civilian targets had been fired at in recent weeks.
Earlier the Ukrainian government said it had evidence three people were operating the missile launcher that brought down MH17 over rebel-held territory near the Russian border.
Officials in the Ukrainian capital, Kiev, said two men had been captured near the border on the Ukrainian side, one of whom had paperwork indicating he was a missile specialist, the ABC reports.
Calls by the Russian and American presidents for a ceasefire in Ukraine's east have been rejected by Alexander Borodai, prime minister of the self-proclaimed Donetsk People's Republic.
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.
Mr Obama called for an immediate ceasefire in eastern Ukraine. He demanded that full access be granted to investigators and that evidence not be tampered with.
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."
Nationalities of MH17 passengers
- Netherlands: 189
- Malaysia: 44 (including 15 crew)
- Australia: 28*
- Indonesia: 12 (including 1 infant)
- UK: 9
- Germany: 4
- Belgium: 4
- Philippines: 3
- Canada: 1
- New Zealand: 1
- Unverified: 4
Source Malaysia Airlines