A US envoy has told the UN the missile that downed a Malaysian airliner is likely to have been fired from a separatist-held area in eastern Ukraine.
Samantha Power added that the US cannot rule out the possibility of help from Russia in the launch of the SA-11 surface-to-air missile that shot down the Malaysia Airlines jet killing all 298 people on board.
Ms Power said the US believes the missile was fired from an area controlled by pro-Russian separatists who had previously boasted about obtaining such missiles, and added that Moscow has provided SA-11s and other heavy weapons to the rebels.
She told an emergency meeting of the UN Security Council: "If indeed Russian-backed separatists were behind this attack on a civilian airliner, they and their backers would have good reason to cover up evidence of their crime.
"Thus it is extremely important than an investigation be commenced immediately."
The Security Council earlier called for "a full, thorough and independent international investigation".
Ukraine has called for an international probe to determine who attacked the plane and insisted it was not its military. Officials accused separatist rebels of shooting down the plane.
The rebels denied it and claimed government forces were behind the crash, which was denied by Ukrainian president Petro Poroshenko.
The Security Council statement expressed "deepest sympathies and condolences to the families of the victims, and to the people and governments of all those killed in the crash".
Council members stood in a moment of silent tribute to the 298 victims at the start of the emergency council meeting, called by Jordan.
The council called for an investigation "in accordance with international civil aviation guidelines and for appropriate accountability". It stressed the need for "immediate access by investigators to the crash site to determine the cause of the incident".
It stressed the need for "immediate access by investigators to the crash site to determine the cause of the incident".
Nataliya Bystro, a spokeswoman for Ukraine's emergency services, said rebel militiamen were interfering with the recovery operation.
The crash site is spread out between two villages in eastern Ukraine with pro-Russia separatists apparently controlling access in and out.
Confusion surrounds the fate of the plane's flight recorders after conflicting reports over whether they had been found.
An assistant to the insurgency's military commander, Igor Girkin, said earlier that eight out of the plane's 12 black boxes had been located and Mr Girkin was considering whether to give international crash investigators access to the crash site.
But another separatist leader, Aleksandr Borodai, said later: "No black boxes have been found. We hope that experts will track them down and create a picture of what has happened."
Russian president Vladimir Putin blamed Ukraine for the downing, saying Kiev was responsible for the unrest in its Russian-speaking eastern regions - but did not directly accuse Ukraine of shooting the plane down and did not address the key question of whether Moscow supplied the rebels with such a powerful missile.
Ukraine and the West have accused Russia of supporting the insurgents, a charge Moscow denies.
Australian prime minister Tony Abbott demanded an independent inquiry into the downing, adding: "The initial response of the Russian ambassador was to blame Ukraine for this and I have to say that is deeply, deeply unsatisfactory.
"It's very important that we don't allow Russia to prevent an absolutely comprehensive investigation so that we can find out exactly what happened here.
"This is not an accident, it's a crime."
Australia lost at least 28 people in the disaster.
Ukraine's ambassador to Nato, Ihor Dolhov, said Kiev has evidence from intercepted phone calls proving pro-Russia rebels caused the crash.
But Russian foreign minister Sergey Lavrov dismissed Kiev's accusations that Moscow itself could have been behind the attack.
"Regarding those claims from Kiev that we allegedly did it ourselves: I have not heard a truthful statement from Kiev for months," he told the Rossiya 24 television channel.
Anton Gerashenko, an adviser to Ukraine's interior minister, said on his Facebook page the plane was flying at about 33,000ft when it was hit by a missile from a Buk launcher, which can fire up to an altitude of 72,000ft. He said only that his information was based on "intelligence".
The Interior Ministry later released a video purporting to show a truck carrying the missile launcher they say was used to fire on the plane with two of its four missiles apparently missing.
The ministry said the footage was filmed by a police surveillance squad this morning as the truck was headed to the city of Krasnodon, towards the Russian border.
The area has seen heavy fighting between government troops and pro-Russia separatists, and rebels had bragged about shooting down two Ukrainian military jets in the region just a day earlier.
Ukrainian prime minister Arseniy Yatsenyuk described the downing as an "international crime" whose perpetrators would have to be punished in an international tribunal.
"Yesterday's terrible tragedy will change our lives. The Russians have done it now," he was cited as saying by the Interfax-Ukraine news agency.
Mr Borodai said 17 representatives from the Organisation for Security and Co-operation in Europe and four Ukrainian experts had travelled into rebel-controlled areas to begin an investigation into the attack.
The militia allowed them to look at part of the crash site but refused to let them view the area where the engines came down.
Russia's ambassador to the UN questioned why Ukrainian aviation authorities allowed a passenger flight through an area of armed clashes where anti-aircraft systems were working.
Vitaly Churkin said there should be an impartial and open investigation into what happened, and the outcome should not be prejudged.
He added that ensuring the security of civilian aviation in a state's airspace is the responsibility of that state.
Ms Power said a Western reporter had earlier reported an SA-11 system in separatist-controlled territory "and separatists were spotted hours before the incident with an SA-11 SAM system close to the site where the plane came down".
"Separatists initially claimed responsibility for shooting down a military transport plane, and claimed responsibility and posted videos that are now being connected to the Malaysia Airlines crash," she added.
"Separatist leaders also boasted on social media about shooting down a plane, but later deleted these messages.
"Because of the technical complexity of the SA-11, it is unlikely that the separatists could effectively operate the system without assistance from knowledgeable personnel. Thus, we cannot rule out technical assistance from Russian personnel in operating the systems."
She said Ukraine also has SA-11 missiles but the US is not aware of any in the area where the plane was shot down.
She said the downing of the jet also follows a pattern of attacks on aircraft by the separatists.
Ukraine's ambassador to the UN said the downing of the airliner would not have happened if Russia had not provided sophisticated anti-aircraft systems to the separatists.
Yuriy Sergeyev told the emergency meeting of the Security Council that communications and intercepts, photos and videos indicate that the rebels have at least two SA-11 missile systems.
He added that detained rebels have told Ukraine they also have a Buk missile system from Russia.
Mr Sergeyev said that immediately after the crash, a rebel military leader boasted on social media of shooting down what he thought was a Ukrainian jet.
He also said that in previous days, rebels had shot down two Ukrainian plane, and claimed responsibility in intercepted telephone conversations with a Russian military intelligence colonel.