* Observers from 23 countries blocked from entering
* Ukraine had invited OSCE members to send monitors
* Group plans next steps after being stopped at checkpoint
WARSAW/VIENNA, March 6 (Reuters) - 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 Organisation 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 organisation 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 Yanukovich was ousted as Ukrainian president last month, and are surrounding military compounds of the Ukrainian army and navy.
Earlier, Polish Defence Minister Tomasz Siemoniak had said unidentified men in military fatigues had prevented the group from entering Ukraine's Crimean Peninsula after travelling from Odessa by car.
One senior Western diplomat criticised 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.