International investigators' quest to carry out their duties at the crash site of the downed Malaysian airliner hit another roadblock Wednesday, as Ukrainian officials warned of possible land mines near the site.

Ukraine's National Security and Defense Council claims that "terrorists" -- the term it uses to describe rebels -- have set up firing positions and laid mines on the access road to the crash site of Malaysia Airlines Flight 17.

The Ukrainian government and pro-Russian rebels are fighting over control of eastern Ukraine. Dutch investigators in Ukraine did not mention mines but announced Wednesday that unsafe conditions kept their contingent from visiting the crash site for the fourth straight day.

CNN could not independently confirm the veracity of the statement by the Ukrainian officials, though CNN's Nick Paton Walsh traveled to and from the debris field safely Wednesday.

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