Thomas Greminger, the OSCE's permanent council chairman, said a team of OSCE monitors did not have the kind of access that they expected. "They did not have the freedom of movement that they need to do their job. The crash site is not sealed off," he said.
"In the current circumstances, they were not able to help securing this corridor that would allow access for those that would want to investigate," Greminger, who is Switzerland's Ambassador to the OSCE, Europe's rights and security watchdog based in Vienna, told Reuters.
OSCE observers visited part of the crash site for a third day on Sunday. Just before their arrival, emergency workers found parts of three more bodies and put them in black body bags on the side of a road.
World leaders have called for a rapid investigation into the shooting down of the airliner, which could mark a pivotal moment in deteriorating relations between Russia and the West.
There were no survivors from the Malaysia Airlines flight MH17 from Amsterdam to Kuala Lumpur, a Boeing 777. The United Nations said 80 of the 298 aboard were children. The deadliest attack on a commercial airliner, it scattered bodies over miles of rebel-held territory near the border with Russia.
The United States said it was "very concerned" about the way the OSCE monitors had been treated. US Secretary of State John Kerry laid out on Sunday what he said was overwhelming evidence of Russian complicity in the downing of the Malaysian airliner in eastern Ukraine.
In a blitz of US morning news shows, Kerry demanded that Russia take responsibility for actions of allied separatists suspected of shooting down the passenger plane and he expressed disgust over the rebels' "grotesque" mishandling of victims' bodies at the crash site.
Kerry also threatened "additional steps" against Moscow and called on European allies, who have lagged behind Washington in imposing sanctions over the Ukraine crisis, to take Thursday's tragedy as a "wake-up call" to take a tougher stand against Russia.
France, Germany and Britain pressed Russian President Vladimir Putin on Sunday to ensure that separatists in Ukraine allow investigators free access to the site of the Malaysian Airlines MH17 crash, the French president's office said.
EU ministers should be ready to announce a fresh round of sanctions at a meeting of the European Union's Foreign Affairs Council this week, said a statement from British Prime Minister David Cameron's office, issued after telephone calls with French President Francois Hollande and German Chancellor Angela Merkel.
Austrian Foreign Minister Sebastian Kurz told the news daily Österreich that "more stringent sanctions on Russia" would be likely if it was confirmed that the plane had been shot down by separatists allied with Russia.