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Russia is facing international pressure to release Ukrainian sailors and vessels it seized Sunday.

READ MORE: Russia attacks and seizes 3 Ukrainian ships near annexed Crimea

NATO head Jens Stoltenberg said Monday that there is no justification for Moscow’s actions.

“All allies expressed full support for Ukraine’s territorial integrity and sovereignty,” Stoltenberg said.

“There is no justification for the use of military force against Ukrainian ships and naval personnel so we call on Russia to release immediately the Ukrainian sailors and ships it seized yesterday.” Tweet This

Ukraine has accused Russia of military aggression and puts its armed forces on full combat alert after Russia on Sunday fired on and then captured the three ships and 24-strong crew in the Kerch Strait, which links the Black Sea with the Azov Sea.

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WATCH: Nikki Haley warns that Russia, Ukraine incident a ‘dangerous escalation’

0:57 Russia, Ukraine incident a ‘dangerous escalation’: Haley Russia, Ukraine incident a ‘dangerous escalation’: Haley

U.S. Ambassador to the United Nations Nikki Haley warned Russia on Monday that its seizure an “outrageous violation of sovereign Ukrainian territory” and urged Moscow to reduce tensions caused by its “arrogant” act.

Haley said she had spoken with President Donald Trump and Secretary of State Mike Pompeo earlier on Monday and that her statement “reflects the concerns at the highest level.”

“As President Trump has said many times, the United States would welcome a normal relationship with Russia,” said Haley, a member of Trump’s cabinet.

“But outlaw actions like this one continue to make that impossible.”

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Russia seized two small Ukrainian armored artillery vessels and a tug boat, which Moscow said had illegally entered Russia’s territorial waters. Kiev said its vessels did nothing wrong and has accused Russia of military aggression.

“The United States will maintain its Crimea-related sanctions against Russia. Further Russian escalation of this kind will only make matters worse. It will further undermine Russia’s standing in the world. It will further sour Russia’s relations with the U.S. and many other countries,” Haley said.

WATCH: Ukrainian nationalists vandalise Russian consulate

1:09 Ukrainian nationalists vandalise Russian consulate Ukrainian nationalists vandalise Russian consulate

Ukrainian U.N. Ambassador Volodymyr Yelchenko called on the international community “to implement a new set of sanctions aimed at addressing the situation in the region, including against Russia’s Azov ports.”

He told the council that increased political pressure on Moscow would help de-escalate the situation, but also warned that Ukraine was “ready to use all available means in exercising our right to self-defense.”

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Russian Deputy U.N. Ambassador Dmitry Polyanskiy accused Ukraine of planning the incident and suggested it was because President Petro Poroshenko’s popularity among voters was low ahead of an election next year.

READ MORE: Ukrainian foreign minister says government determined to take back Crimea from Russia

“How can he maintain power in these circumstances? It’s clear – organize provocation and once again accuse Russia of everything, inflate his own ratings and put himself forward as the savior of the nation,” Polyanskiy told the council.

“This is about canceling the elections despite all of Poroshenko’s assurances to the contrary,” he said, warning that Russia “has never caused the first blow, but it knows how to protect itself.”

Yelchenko rejected Polyanskiy’s remarks about motives. He told the council the incident was a “clear threat to international peace and security” and that Russia was mixing reality with fiction in trying to explain what happened.

U.N. political affairs chief Rosemary DiCarlo told the Security Council the United Nations could not independently verify what had happened.

“We strongly urge both the Russian Federation and Ukraine to refrain from any ratcheting up of actions or rhetoric and remind both of the need to contain this incident so as to prevent a serious escalation,” she said. Tweet This

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The 15-member Security Council has met dozens of times over the crisis in Ukraine since Russia annexed Crimea in 2014. It is unable to take any action as Russia is one of the council’s five veto powers.

The Ukrainian vessels seized by Russia on Sunday had been trying to enter the Sea of Azov from the Black Sea via the narrow Kerch Strait that separates Crimea from the Russian mainland.

— Reporting by Michelle Nichols Editing by James Dalgleish