The OSCE observers were accompanied by armed guards to the town of Torez near the crash site and inspected three to four refrigerated train carriages loaded with bodies of the victims there, Xinhua quoted Michael Bochurkiv as saying.
He added that it was impossible to say accurately how many dead bodies had been found.
A Boeing 777 passenger plane of the Malaysia Airlines crashed in eastern Ukraine en route from Amsterdam to Kuala Lumpur on Thursday, killing all 298 people aboard.
An OSCE monitoring mission arrived at the scene on Friday and 132 experts from Malaysia arrived in Kiev on Saturday.
US State Department spokeswoman Jen Psaki said earlier that the US was "deeply concerned" by rebels' refusal to allow monitors safe and unfettered access to the airliner crash site.
Officials say impossible to give accurate number of bodies found
Alexander Borodai, prime minister of the self-proclaimed Donetsk People's Republic (DPR), said the alleged denial of OSCE monitors' entry into the site is "an element of information war against us".
"Fourteen OSCE observers have already been working at the crash site for three days," he said in an interview, adding there were indeed some restrictions on their movement on the first day for security reasons.
Ukrainian officials are in talks to move the loaded train from the Donetsk region to Kiev-controlled territory, the Interfax news agency quoted Ukrainian Deputy Prime Minister Vladimir Groisman as saying.
He also said authorities in Kiev cannot guarantee security to foreign experts working at the site since the territory is controlled by rebels.
He noted, however, Kiev is ready to help experts reach the site, as Ukrainian President Petro Poroshenko promised to take them by plane to a safe area nearest to the site.
According to Groisman, as many as 223 bodies of victims have been retrieved from the crash site.