There are hopes that a credible investigation can be conducted by international experts. There are also concerns that evidence has been compromised before inspectors can reach the scene.
The Ukrainian government says pro-Russian militiamen have removed 38 bodies from the crash site and taken them to the rebel-held city of Donetsk. Kiev also claims the rebels are "seeking large transports to carry away plane fragments to Russia."
A separatist leader denies that any bodies have been transferred or that the rebels have interfered with the work of observers.
It's not clear whether anyone has recovered the plane's flight recorders.
Ukraine and the separatists accuse each other of shooting down the Boeing 777 with a surface-to-air missile.
Flight 17 was flying from Amsterdam to Kuala Lumpur, Malaysia, with 283 passengers and a crew of 15. The victims came from 13 nations. They included 192 Dutch citizens and 80 children.