A team of 41 military observers, including two Britons, from the Organization for Security and Co-operation in Europe (OSCE) have been stopped from entering Crimea by unidentified men in military fatigues, the OSCE said.
"The mission has been detained, they cannot go further. They landed in Odessa and they were travelling by car from Odessa towards the Crimean Peninsula, but they were detained by unidentified men in fatigues," Poland's defence minister, Tomasz Siemoniak, told reporters.
He said it was possible the observers would be allowed to head back the way they came, but they could not go forward into Crimea.
Two Polish military officers are among the OSCE mission.
Barack Obama warned Russia that the West will be forced to apply a cost to Moscow if it fails to change course in its dispute with Ukraine.
The ballot paper that the people of Crimea will use in Sunday's referendum was published today
Vitali Klitschko's visit to the eastern city of Donetsk is clearly an attempt to try to ease the divisions with Ukraine.