Ukraine's border service has said that it will only allow Ukrainian citizens to travel to Crimea following the imposition of martial law.
Kyiv imposed martial law in 10 of its 27 regions for 30 days on November 28 after Russian forces fired on Ukrainian ships and seized 23 sailors in the Black Sea off the coast of the Russian-controlled Crimean Peninsula.
"In connection with the introduction of martial law, the administrative border with temporarily occupied Crimea can be crossed exclusively with Ukrainian documents," a spokesman said on November 29.
Citizens from all nations were previously allowed to enter Crimea through the administrative border via mainland Ukraine. But the process for doing so for non-Ukrainians was fraught with bureaucracy.
Crimea is accessible by plane from Russia or via Russia's newly built bridge from the country's mainland. But under Ukrainian law, those routes are illegal. Violators -- and there have been many -- are given official bans of three years or longer by Kyiv.
Earlier in the day, Ukrainian President Petro Poroshenko called on NATO to send ships to the Sea of Azov to help protect Ukraine.
He claimed Russian President Vladimir Putin sees himself as a "Russian emperor" and Ukraine as a Russian "colony."
The Ukrainian president later on November 29 tweeted that Kyiv will impose "restrictions" on Russian citizens in Ukraine.
"No need to run to shops and buy matches and salt. There will be no restrictions on cash withdrawals, currency-exchange operations, travels abroad for Ukrainian citizens. For Russian citizens, these restrictions will be introduced. And I think that's quite justified," he wrote.
Relations between Moscow and Kyiv have deteriorated after Russia seized Ukraine's Crimean Peninsula in March 2014 and shortly thereafter began supporting separatists in eastern Ukraine in a conflict that has left more than 10,300 dead since April 2014.
In an interview with the German tabloid Bild published early on November 29, Poroshenko said he hoped European states will take active steps, including increasing sanctions and military protection against Russia, to help Ukraine after providing verbal support in the wake of Russia's capture of 24 Ukrainian sailors over the weekend.
"We hope that NATO states are prepared to send naval ships to the Sea of Azov to support Ukraine and provide security," Poroshenko said.
"The only language he [Putin] understands is the solidarity of the Western world," Poroshenko said. "We can't accept Russia's aggressive policies. First it was Crimea, then eastern Ukraine, now he wants the Sea of Azov."
Opening a German-Ukrainian economic forum in Berlin later in the day, German Chancellor Angela Merkel said she planned to press Putin at a Group of 20 (G20) summit later this week to urge the release of the ships and crews.
"We can only resolve this in talks with one another because there is no military solution to all of these conflicts," she added.
Meanwhile, an unknown number of the captured Ukrainian sailors have since been transferred to a detention center in Moscow, according to one of their lawyers.
Dzhemil Temishev wrote on Facebook on November 29 that his "colleagues" in the Lefortovo detention center in Moscow had informed him that some of the Ukrainian sailors had been brought there.
Also on November 29, Kremlin spokesman Dmitry Peskov criticized Poroshenko’s request for NATO to deploy naval ships to the Sea of Azov, saying it was "aimed at provoking further tensions" and driven by Poroshenko’s "electoral and domestic policy motives."
Putin has claimed that the naval confrontation was a ploy to boost his Ukrainian counterpart's popularity ahead of an election in March.
A NATO spokeswoman said the alliance already had a strong presence in the region, with vessels routinely patrolling and exercising in the Black Sea.
"There is already a lot of NATO in the Black Sea, and we will continue to assess our presence in the region," Oana Lungescu said.
The Sea of Azov is the body of water that separates the Crimean Peninsula, which Russia illegally annexed in 2014, from the Ukrainian and Russian mainlands. Russia opened a bridge over the Kerch Strait connecting Crimea with Russia in May and has asserted control over the strait.
The Kerch Strait is the only route for ships traveling between the Sea of Azov, where Ukraine has several ports, and the Black Sea, which is the arena usually patrolled by NATO.
Ukraine is a partner of NATO but not a member of the military alliance. NATO has already said it "stands with Ukraine" and has called on Russia to release the captured ships and their crews.
NATO chief Jens Stoltenberg also warned Russia on November 26 that "its actions have consequences."
Poroshenko, who on November 28 instituted martial law in parts of Ukraine in response to the Russian actions, told Bild he had evidence suggesting Russia is planning a new ground offensive against Ukraine, and he said he had shown NATO partners satellite pictures supporting that allegation.
"Germany also has to ask itself: What will Putin do next if we don't stop him?" Poroshenko told Bild.
Ukrainian parliament speaker Andriy Parubiy has joined Poroshenko in calling for increased protection from NATO, saying on November 27 that "I urged [NATO] to increase [its presence] in the airspace above the Black Sea and the number of military ships in the Black Sea as a guarantee of security and a guarantee of stability in the Black Sea."
EU: 'Utmost Concern,' But No New Sanctions
Poroshenko's remarks came as the European Union failed to muster support for any immediate new steps to either impose new sanctions on Russia over the naval incident or increase enforcement of existing sanctions on Moscow.
Poland, Britain, and the EU's Baltic states have called for more sanctions, but after three days of debate, the EU's 28 states could agree only to issue a statement on November 28 expressing "utmost concern about the dangerous increase of tensions" and the "unacceptable" use of force by Russia.
The statement issued by EU foreign-policy chief Federica Mogherini did not mention sanctions, saying only that the bloc will "act appropriately" while continuing to monitor the situation.
The bloc's top powers, Germany and France, have so far emphasized efforts to ease tensions. Other members, including Italy, Greece, Bulgaria, and Cyprus, have been calling for a softening of sanctions on Moscow.
The EU first imposed sanctions on Russia after it seized Crimea, and it has ratcheted up those sanctions from time to time. The United States on November 27 called for stricter enforcement of the EU's existing sanctions on Russian and Crimean officials and businesses.
While the EU failed to take any immediate action against Russia, in a gesture of solidarity with Ukraine, Estonia said on November 28 that it had summoned its Russian ambassador and condemned Russia's use of military force in the incident.