The United States on Monday called Russia's move to seize three Ukrainian vessels in the Sea of Azov near Crimea a "violation of international law."
On Sunday, Russia said the vessels had ignored calls to stop, sparking military action, as they were relocating from Odessa to the Port of Mariupol.
At an emergency meeting of the UN Security Council, U.S. Ambassador to the United Nations Nikki Haley condemned Russia for its "outrageous violation of sovereign Ukrainian territory."
"This is no way for a law-abiding, civilized nation to act. Impeding Ukraine’s lawful transit through the Kerch Strait is a violation under international law," said Haley. "It is an arrogant act that the international community must condemn and will never accept."
The Ukrainian navy accused Russia of opening fire on its warships off Crimea, asserting that crew members were wounded and its vessels were seized by Russia.
"The United States will continue to stand with the people of Ukraine against this Russian aggression," she said.
"We call on Russia to respect its international obligations and not obstruct or harass Ukraine’s transit in the Kerch Strait and to de-escalate the tension it has created," said the ambassador, adding that Washington would welcome "a normal relationship" with Moscow.
"But outlaw actions like this one continue to make that impossible," she added.
She warned that Russian escalation of this kind will further sour Russia’s relations with the U.S. and many other countries and further increase tensions with Ukraine.
Also, NATO's chief also called on Moscow to immediately free Ukrainian ships and soldiers seized in the confrontation on Sunday.
“NATO allies support Ukraine's territorial integrity and sovereignty. There is no justification for the use of military force against Ukrainian ships and naval personnel. We urge Russia to free Ukrainian ships and soldiers at once,” Secretary-General Jens Stoltenberg said at a news conference after a NATO-Ukraine emergency meeting in Brussels.
"We consider such steps as a violation Russia's sovereignty," said Russian envoy Dmitry Polyansky, adding that Sunday's incident was planned in advance and backed by the Western states.
Polyanskiy said the UN Security Council should take responsibility of Ukraine's "dangerous acts."
Ukraine's UN envoy, Volodymyr Yelchenko, for his part, said the Kremlin's "criminal regime has once again demonstrated that it won't stop its aggressive policy and is ready for any act of aggression against Ukraine state."
The "dramatic escalation" of the situation in the region needs prompt action from the international community, said Yelchenko.
Russia and Ukraine have been at loggerheads since 2014, when Russia annexed Crimea after a controversial referendum.
Turkey, as well as the UN General Assembly, viewed the annexation as illegal.
Ukraine has also blamed Russia for separatist violence in eastern Ukraine, near the border with Russia.