Ukraine's Prime Minister Arseniy Yatsenyuk has said he will go to Washington this week to discuss the crisis with Russia over Ukraine's region of Crimea.
Yatsenyuk's remarks come as Russia appears to be tightening its military grip on the peninsula.
Earlier, Ukraine's State Border Service said pro-Russian forces took control of a Ukrainian border guard post in western Crimea early on March 9, leaving around 30 Ukrainian border guards trapped.
Spokesman Oleh Slobodyan said the Chernomorske base on the far western coast of the Crimean Peninsula was seized without bloodshed at around 6:00 in the morning local time.
It said such forces now control 11 border-guard posts in Crimea.
Pro-Moscow authorities there have voted to split from Ukraine and join Russia and called for a 16 March referendum to confirm that decision.
Both moves have been dismissed in Kyiv and Washington and other Western capitals as illegitimate.
Yatsenyuk vowed earlier on March 9 that Ukraine will not give "an inch" of its territory to Russia.
Pro-Russian activists meanwhile attacked pro-Ukrainian activists with clubs and whips in the Crimean city of Sevastopol as thousands took to the streets across Ukraine in rival demonstrations.
The violence took place as some 200 demonstrators gathered to commemorate the 200th anniversary of poet and national hero Taras Shevchenko.
In Crimea's regional capital, Simferopol, separate rallies in support of Ukrainian unity and for joining Russia proceeded peacefully.
At a rally in the eastern city of Donetsk, former opposition leader Vitaly Klitschko said Ukraine should not be allowed to split apart amid bloodshed.
Thousands of pro-Russian demonstrators also held a rally in that city, protesting outside the regional government building that's been beseiged several times in the past week by pro-Moscow mobs.
U.S. Secretary of State John Kerry has told Russian Foreign Minister Sergei Lavrov that annexing Crimea "would close any available space for diplomacy."
Ukrainian authorities recently put the number of Russian forces in Crimea at 30,000, while the U.S. Pentagon has suggested the number is around 20,000.
European Union leaders have said they will fast-track for "immediate" signing the political chapters of an Association Agreement with Ukraine, the same document that sparked Ukrainian unrest after now-ousted Ukrainian President Viktor Yanukovych decided not to sign in late November.
Based on reporting by Reuters, 5.ua, RFE/RL's Russian and Ukrainian services, and AFP