US President Barack Obama has said evidence so far indicates that Malaysia Airlines Flight 17 carrying 298 people was shot down by a surface-to-air missile from an area in Ukraine controlled by Russian-backed separatists.
Obama said on Friday that it was not the first time the separatists 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 for a full investigation into Thursday's air crash.
"This is an outrage of unspeakable proportions," Obama said.
"An Asian airliner was destroyed in European skies filled with citizens from many countries, so there has to be a credible international investigation into what happened."
Officials from the FBI and the National Transportation Safety Board were on their way to Ukraine to help determine what happened, Obama said.
He said that evidence must not be tampered with as a UN-backed investigation went forward.
"We will hold all its members, including Russia, to their word" in allowing access to the crash site, Obama said.
Malaysia Airlines Flight 17 crashed at Grabovo, about 40km from the border with Russia near the regional capital of Donetsk, an area that is a stronghold of rebels who have been fighting Ukrainian government forces.
The debris rested "in a vast area around wheatfields with a circumference of at least 10km", Al Jazeera's Scott Heidler, reporting from the crash site, said.
Leaders of the rebels' self-proclaimed Donetsk People's Republic denied any involvement and said a Ukrainian air force jet had brought down the intercontinental flight.
Samantha Power, US ambassador to the UN, in a speech at an emergency meeting convened by the Security Council on Friday, said the US could not rule out that Russian personnel had assisted separatists in firing a missile at Flight 17.
The Security Council, after the meeting, called for a "full, thorough and independent international investigation" into the downing of the airliner, urging all parties to grant investigators access to the site.
In a statement agreed by consensus, the council also called for "appropriate accountability" for the incident.
Both Ukraine and the separatists later agreed to establish a corridor through the battlefield to allow international investigators in.
However, about 30 monitors from the Organisation for Security and Cooperation in Europe (OSCE) had arrived at the Grabovo crash site on Friday but said they were unable to inspect the area properly and would try again on Saturday.
Al Jazeera's Nazanine Moshiri, reporting from Dnepropetrovsk, Ukraine, said some investigators were at the site for about 75 minutes but "were unable to make any kind of decent assessment" as a ceasefire was not in place.
"Journalists and observers at the scene could hear the thuds of missiles," she said.
Earlier, Ukraine's government said it had not used anti-aircraft missiles in its fight with pro-Russia rebels in the east of the country.
"All missiles that are in our armoury, not one of them has been used," Andriy Lysenko, spokesman for Ukraine's security council, said.
Bohdan Senyk, a spokesman for the Ukraine Defence Ministry, said the passenger jet was out of range of the Ukrainian army's anti-aircraft missile systems.
"Anti-aircraft missiles have not been deployed during the anti-terrorist operation ... they are all in place," he said.
Russia's Defence Ministry said on Friday it had picked up radar facility activity from a Ukrainian Buk missile system on Thursday.
Ukraine has since closed the airspace over the east of the country.
The victims were from 13 countries, with the majority - 192 - coming from the Netherlands.
In his reaction, Mark Rutte, the Dutch prime minister, said: "When it becomes clear that this was an attack, then I will personally ensure that the perpetrators will be hunted and punished accordingly."