Pro-Russia gunmen have fired warning shots to prevent international military observers as they tried to cross the border and enter Ukraine.
For the third day running inspectors from the Organisation for Security and Cooperation in Europe (OSCE) have been unable to enter the disputed region of Crimea.
The Associated Press reported: "Warning shots fired, foreign military observers barred from entering Crimea. Warning shots of automatic weapons were fired into the air. No injuries were reported."
The team of 43 unarmed inspectors from 23 countries has been turned away twice this week at Crimea's land border with the rest of Ukraine after failing to get past roadblocks.
Yesterday pro-Russia armed men also intimidated pro-Ukrainian citizens in the region, and surrounded a military base in Sevastopol. Pro-Russian soldiers tried to seize another Ukrainian military base outside the biggest city, Sevastopol, overnight but no shots were fired and they pulled back.
Ukraine had invited representatives of the Vienna-based security organisation and democracy watchdog to monitor the situation in Crimea, but the team was forced to abandon the mission and return to the Ukrainian town of Kherson, halfway between Odessa and the Crimean peninsula.
Pro-Russian forces took control of key installations on the peninsula on 28 February in a largely bloodless operation which seems to have had the support of many of Crimea's majority ethnic Russians.
The OSCE 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.