A Ukrainian sailor, right, is escorted by a Russian intelligence agency FSB officer to a court room in Simferopol, Crimea, Tuesday, Nov. 27, 2018. The Russians seized the ships and their crews, who are expected to face a court hearing later Tuesday in the Crimean capital of Simferopol. (AP Photo)

A Ukrainian sailor, right, is escorted by a Russian intelligence agency FSB officer to a court room in Simferopol, Crimea, Tuesday, Nov. 27, 2018. The Russians seized the ships and their crews, who are expected to face a court hearing later Tuesday in the Crimean capital of Simferopol. (AP Photo)

KIEV, Ukraine (AP) — Russia on Tuesday began prosecuting the crew of Ukrainian navy vessels captured over the weekend in a confrontation off Crimea, putting some of the seamen on camera, where they confessed to intruding into Russian waters.

Ukraine demanded that Russia stop using “psychological and physical pressure” on the sailors, as tensions between the two neighbors escalated. Ukraine’s top diplomat called the men “prisoners of war,” telling The Associated Press that displaying them on TV was a crime.

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Russia and Ukraine have blamed each other for Sunday’s clash in the Kerch Strait, which links the Black Sea and the Sea of Azov. The incident has drawn strong criticism of Russia by the United States and its allies and fueled fears of a full-blown conflict in eastern Ukraine.

Ukraine’s parliament on Monday adopted a motion by President Petro Poroshenko to impose martial law for 30 days in parts of the country — a measure that Kiev avoided even when Russia annexed the Crimean Peninsula in 2014 or sent clandestine troops and weapons to separatists in eastern Ukraine.

Kremlin spokesman Dmitry Peskov warned the move could cause hostilities to flare up in eastern Ukraine.

“The introduction of martial law could potentially raise the threat of escalation of tensions in the region of conflict,” he told reporters.

Ukrainian troops have been fighting Russian-backed separatists in eastern Ukraine since 2014, a conflict that has left over 10,000 dead, but fighting has eased since a truce in 2015. Martial law went into effect in several parts of Ukraine, including areas bordering territory held by the separatists.

In Sunday’s confrontation, Ukraine sent three small naval vessels from their Black Sea base in Odessa to Berdyansk on the Sea of Azov. The Russian coast guard blocked their way near the Kerch Strait and at one point rammed one of the Ukrainian vessels to keep it from passing through the narrow waterway. The ships had spent tense hours maneuvering until the Russians opened fire and seized the Ukrainian vessels and crew.

The Ukrainian navy said that six of its seamen were wounded, while Russia said that three Ukrainian crewmen were slightly injured. Ukraine said 24 of its sailors were taken into custody.

Russian state television broadcast separate interviews with three of the seamen, who said that the Russian coast guard repeatedly warned them that they were violating Russia’s territorial waters and urged them to leave. It was not clear if the men were talking under duress, but one was clearly reading from a script on camera.

Ukraine’s National Security Service, or SBU, demanded that Russia stop using “psychological and physical pressure” on the men — an apparent reference to the interviews.

Ukrainian Foreign Minister Pavlo Klimkin told the AP in an interview that he has asked the president of the International Committee of the Red Cross to arrange a visit with the captives and is awaiting a Russian response. He said some of the seamen had been seriously injured in the clash.

“It’s not a political issue here, because we can have an argument about the legal status, but it’s about simply concentrating on protecting them and helping them,” Klimkin said.

When asked about the seamen shown on Russian TV, Klimkin said “even to put prisoners of war on television is already a crime.”

A court in the Crimean regional capital of Simferopol put 12 of the Ukrainians in custody for two months pending trial on charges of violating the Russian border.

One of the seamen, Yuri Budzylo, commented briefly to media while awaiting the court decision, saying his vessel was sent into the Sea of Azov with the understanding that an agreement was in place for free passage for Ukrainian ships.

Ukraine said its vessels were operating in line with international maritime rules, while Russia alleged they had failed to get permission to pass. A 2003 treaty between the two countries designated the Kerch Strait and the Sea of Azov as shared territorial waters, but Russia claimed the strait in its entirety after annexing Crimea and has sought to assert greater control over the passage.

Klimkin said the use of force against the Ukrainian vessels was a “show of strength” by Russia intended to buttress Russian President Vladimir Putin’s falling approval ratings.

“He needs a permanent crisis with Ukraine,” he told the AP.

Declaring martial law was needed to protect Ukraine from a possible Russian invasion, Klimkin said.

But Russian officials and state media cast the incident and the imposition of martial law as an attempt to shore up Poroshenko’s own sagging popularity ahead of a presidential election in March.

“Martial law allows the current government to flex its muscles and try to bolster public support,” Russian Prime Minister Dmitry Medvedev said. “Or they could be looking at cancelling the election as an ultimate move.”

NATO on Tuesday emphasized its “full support to Ukraine’s sovereignty and territorial integrity,” noting there is no justification for Russia’s use of military force against Ukrainian ships. It urged Russia to release the Ukrainian seamen and vessels promptly.

Russia insisted it was protecting its waters against an illegal intrusion.

Russian Foreign Minister Sergey Lavrov noted that Ukrainian ships in the past had sought and quickly got permissions to pass through the strait.

Speaking on a visit to Paris, Lavrov accused Ukraine of a deliberate “provocation” and urged the West to warn Ukraine’s leadership against “playing with fire.”

Putin spoke to German Chancellor Angela Merkel early Tuesday, expressing “serious concern” about what martial law in Ukraine might entail.

German Foreign Minister Heiko Maas said Berlin has urged both Russia and Ukraine “to show the greatest possible restraint” and suggested that Germany, France, Russia and Ukraine could work together to resolve the tensions.

Lavrov rejected that offer, saying he did not see “a need for any kind of mediators.” He spoke after meeting with French Foreign Minister Jean-Yves Le Drian, who appeared to soften his criticism of Moscow.

France’s Foreign Ministry said shortly after the incident that “nothing justifies” Russia’s use of force, but after long talks with Lavrov, Le Drian blamed the standoff on the “high level of militarization” in the region and avoided pointing the finger at Russia.

The U.S. and the European Union have slapped sanctions on Russian businesses, tycoons and banks over the annexation of Crimea.

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Vasilyeva reported from Moscow. Yuras Karmanau in Minsk, Belarus, Vladimir Isachenkov in Moscow, Angela Charlton in Paris, and Kirsten Grieshaber and Geir Moulson in Berlin contributed.

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Read all about the Russia-Ukraine story on The Associated Press website at https://www.apnews.com/Ukraine