The source would not specify exactly where or when the team of 40 observers from the Organisation for Security and Cooperation would make the attempt.
There are only two main roads leading into Crimea from the rest of Ukraine, one crossing the narrow Isthmus of Perekop in the west and another eastern route on the Azov Sea.
Near the village of Chonhar on the eastern route, AFP reporters said about 10 armed men were manning a checkpoint on the road, flagging down and checking passing vehicles. The road was partially blocked by concrete slabs.
The observers, from 21 of the organisation's 57 members, spent the night in the Ukrainian city of Kherson near Crimea after gunmen prevented them from crossing one of the roads into the peninsula.