Pro-Russian groups have seized two Ukrainian warships in Sevastopol.
Ukraine’s Defence Ministry claimed the vessels were attacked with grenades.
There are no reports of any casualties.
Tensions are high in the area after irregular pro-Russian militia entered Ukraine’s naval headquarters on Wednesday.
Many Ukranian soldiers have defected or deserted.
In Geneva Ukraine’s ambassador to the UN Yuri Klymenko warned of a sharp deterioration in relations between Ukraine and Russia.
Quoted by the Associated Press, he said that he believed Russia was preparing for a military “invasion” in more areas of his country.
Kyiv has sent a delegation to Sevastopol.
The Crimean authorities say they will be refused entry into the territory.