Alexander Hug, deputy head for the Organization for Security and Cooperation in Europe's monitoring mission in Ukraine, said: "We heard indications there's fighting going on.
"The situation on the ground appears to be unsafe ... we therefore decided to deploy tomorrow morning," he said, flanked by Australian and Dutch investigators.
The eastern regions of Donetsk and Luhansk are in the grip of heavy fighting as Ukrainian government forces try to dislodge the pro-Russian separatists.
"Fighting in the area will most likely affect [the] crash site," Hug said.
The separatists remain in control of the area where Malaysia Airlines flight MH17 was shot down earlier this month and have restricted access.
Earlier, Malaysian premier Najib Razak said an agreement reached with separatist leader Aleksander Borodai would "provide protection for international crash investigators" to recover bodies and ascertain the cause of the crash.