WARSAW/VIENNA — Dozens of military observers invited by Ukraine to monitor the situation in Russian-occupied Crimea had to turn back on Thursday after failing to get past roadblocks, the Organization for Security and Cooperation in Europe said.

The mission of 43 unarmed observers from 23 OSCE countries was now heading back to the Ukrainian town of Kherson, halfway between Odessa and the Crimean peninsula, to decide how to proceed, the Vienna-based security organization and democracy watchdog said.

"They tried two checkpoints, they didn't get through ... Nobody attacked them, they were just not allowed to go through," an OSCE spokeswoman said.

The mission had aimed to defuse a tense military standoff. Forces loyal to Moscow bloodlessly seized Crimea after Russian ally Viktor Yanukovych was ousted as Ukrainian president last month, and are surrounding military compounds of the Ukrainian army and navy.

Earlier, Polish Defense Minister Tomasz Siemoniak had said unidentified men in military fatigues had prevented the group from entering Ukraine's Crimean Peninsula after traveling from Odessa by car.

One senior Western diplomat criticized the obstacles.

"It's not surprising but it is obviously unacceptable and it's unfortunate given the mission they were sent out to do," he said.