Tensions in the region remained high despite the release of a Ukrainian naval commander held by pro-Russian forces.
Shots were fired but there were no casualties as the Ukrainian corvette Khmelnitsky was seized in Sevastopol, according to an AP photographer at the scene. Another ship, the Lutsk, was also surrounded by pro-Russian forces.
The Defense Ministry had no immediate information on the incidents.
Ukraine's Deputy Defense Minister Leonid Polyakov accused Russian troops of constantly threatening to storm military bases where Ukrainian soldiers were located, according to the Interfax news agency.
In Geneva, Ukraine's ambassador to the United Nations warned of a sharp deterioration in relations between the two neighbors, saying that Russia appears to be preparing for a military "invasion" in more areas of his country.
Ambassador Yuri Klymenko said there were "indications that Russia is on its way to unleash a full-blown military invention in Ukraine's east and south" since its annexation of Crimea. He said his statement was based on information from non-governmental organizations.
In an address to the German Parliament in Berlin, Chancellor Angela Merkel said the EU was readying further sanctions and that the G-8 forum of leading economies had been suspended indefinitely. Russia holds the presidency of the G-8 and President Vladimir Putin was due to host his counterparts, including President Barack Obama, at a G-8 summit in Sochi in June.
"So long as there aren't the political circumstances, like now, for an important format like the G-8, then there is no G-8," Merkel said. "Neither the summit, nor the format."
The EU and the United States have slapped sanctions on individuals involved in what they say was Crimea's unlawful referendum over joining Russia. Moscow formally annexed Crimea earlier this week in the wake of the poll. The Black Sea peninsula had been part of Russia for centuries until 1954 when Soviet leader Nikita Khrushchev transferred it to Ukraine, and many residents were happy about rejoining Russia.
Russian forces effectively took control of Crimea some two weeks ago in the wake of the ouster of Ukraine's pro-Russia president, Viktor Yanukovych, after months of protests and sporadic violence. The crisis erupted late last year after Yanukovych backed out of an association deal with the EU in favor of a promised $15 billion bailout from Russia. That angered Ukrainians from the country's pro-European central and western regions.
Merkel said EU leaders would increase those "level 2" sanctions against Russia when they meet later Thursday in Brussels to widen the list of those whose assets are being frozen and who are banned from traveling.
She also reiterated that if things worsen, the EU is prepared to move to "level 3" measures, which would include economic sanctions.
"The European Council will make it clear today and tomorrow that with a further deterioration of the situation we are always prepared to take level 3 measures, and those will without a doubt include economic sanctions," she said.
Merkel's tough approach came as the commander of Ukraine's navy was freed after being held by Russian forces and local Crimean militia at the navy's headquarters in Crimea.
Rear Adm. Sergei Haiduk and an unspecified number of civilians were held for hours after the navy's headquarters in Sevastopol was stormed Wednesday. Acting Ukrainian President Oleksandr Turchynov, who confirmed the release, said Russian forces were involved in the storming.
In response, Ukraine on Wednesday said it would hold joint military exercises with the United States and Britain, signatories, along with Russia, of the 1994 Budapest Memorandum - a document designed to guarantee Ukraine's territorial integrity when it surrendered its share of Soviet nuclear arsenals to Russia after the Soviet Union broke up in 1991.
With thousands of Ukrainian soldiers and sailors trapped on military bases, surrounded by heavily armed Russian forces and pro-Russia militia, the Kiev government said it was drawing up plans to evacuate its outnumbered troops from Crimea back to the mainland and would seek U.N. support to turn the peninsula into a demilitarized zone.
Just how many retreating troops Ukraine will have to absorb in what amounts to a military surrender of Crimea was unclear. Many servicemen have already switched sides to Russia, but authorities said they were prepared to relocate as many as 25,000 soldiers and their families to the Ukrainian mainland.
At Belbek airbase in the wine-growing country near Crimea's southwestern coast, airmen were leaving Thursday toting plastic shopping or garbage bags filled with their belongings.
They weren't evacuating, they said, just transferring their things to a safe place. They were worried that pro-Russian mobs might loot the facility, as they heard happened the day before in nearby Sevastopol.
Since Russian forces took charge in Crimea, Ukrainian-enlisted personnel and officers have been bottled up in barracks and other buildings at one end of the Belbek base, with the Russians in control of the airfield.
"We're waiting for what Kiev, our leadership, tells us," said one major, who declined to give his name. The major said he expected about half of the personnel still at the base to accept the Russian offer to stay and join the Russian armed forces, since they were Crimea natives.
Humbled but defiant, Ukraine lashed out symbolically at Russia by declaring its intent to leave the Moscow-dominated Commonwealth of Independent States, a loose alliance of 11 former Soviet nations. The last nation to leave the group was Georgia, which fought a brief war with neighboring Russia in 2008 and ended up losing two separatist territories.
UN Secretary-General Ban Ki-moon was visiting Moscow on Thursday and was to come to Ukraine on Friday.
"We are working out a plan of action so that we can transfer not just servicemen, but first of all, members of their family who are in Crimea, quickly and effectively to mainland Ukraine," said Andriy Parubiy, secretary of Ukraine's National Security and Defense Council.
Rising contributed from Berlin. John-Thor Dahlberg in Belbek, Crimea and John Heilprin in Geneva also contributed to this report.