It's after Russia seized three of its vessels.
Ukraine's parliament voted on Monday to introduce martial law for 30 days in areas of the country most vulnerable to an attack from Russia.
The decision came after Russia on Sunday seized two small Ukrainian armoured artillery vessels, a tugboat and some 23 crewmembers, which it said had illegally entered its territorial waters.
The vote came after Ukraine's National Security and Defense Council (NSDC) "unanimously supported" a proposal from President Petro Poroshenko to invoke 60 days of martial law following the incident.
A statement on the NSDC website said it "considered it necessary to introduce a special legal state of martial law in Ukraine in order to create conditions for repelling armed aggression, to ensure national security, and to eliminate threats to the state independence and territorial integrity of Ukraine."
Martial law is typically instated during a temporary crisis, and can impose military control on state institutions and restrict civil liberties.
How did we get here?
Russia on Sunday seized the three vessels, which were sailing from Ukraine’s Black Sea port of Odesa to Mariupol in the Sea of Azov when they were confronted by Russian ships in an “openly aggressive” manner, the Ukrainian Navy said in a statement.
The vessels were approaching the Kerch Strait when the tugboat was struck, damaging its engine, guardrail and outer shell.
Footage thought to have been taken during the skirmish has been shared on social media.
“The Russian Federation has once again demonstrated its aggressive nature and complete neglect of the norms of international law,” the Navy said, referring to a treaty from 2003 which guarantees free passage through the Kerch Strait for Ukrainian and Russian vessels.
Ukraine’s foreign ministry added that it would liaise with its allies concerning Moscow’s “violent acts”, saying they “pose a threat to the security of all states of the Black Sea region, and therefore require a clear reaction from the international community”.
Russia's Federal Security Service, however, says Kyiv “illegally” entered its territorial waters and carried out “dangerous manoeuvres” against the orders of Russian authorities, prompting them to act.
Passage through the Kerch Strait had been “temporarily blocked” by a Russian cargo ship sitting under a Moscow-controlled bridge “for security reasons” when the incident unravelled, it added.
Ukraine's President demanded the release of Ukrainian sailors in a statement early on Monday morning.
"I appeal to the leadership of the Russian Federation with a demand to immediately release the Ukrainian servicemen who were brutally detained in violation of international law and whose fate is unknown," he said during a meeting of the National Security and Defense Council.
"We demand to immediately transfer them to the Ukrainian side along with the ships and ensure de-escalation of the situation in the Sea of Azov."
Poroshenko summoned his war cabinet in the aftermath of Sunday's attack, which Russia's Federal Security Service (FSB) said was sparked by Ukraine and that their "irrefutable" evidence of the fact "will soon be made public".
NATO is calling for calm and its head Jens Stoltenberg urges Russia to release the Ukrainian navy ships together with its sailors. NATO said there was no justification for Moscow's actions.
After emergency talks, Stoltenberg told a news conference of the Western military alliance held at the request of Ukraine, which is not a NATO member, "We saw that Russia used military force against Ukraine in an open and direct way," and added "All allies expressed full support for Ukraine's territorial integrity and sovereignty." He also 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."
The US ambassador to the United Nations, Nikki Haley, warned Russia on Monday that its seizure of three Ukrainian vessels was an "outrageous violation of sovereign Ukrainian territory", as she called on Moscow to reduce tensions caused by its "arrogant" act.
Haley made the comments at a UN Security Council session, explaining that she had spoken with US President Donald Trump 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. But outlaw actions like this one continue to make that impossible," she said.
Meanwhile, President Trump says he is not liking what is happening between Russia and Ukraine. He says he's working with European leaders to find a solution.
The rise in tensions also spurred the European Union to intervene. The bloc called on Kyiv and Moscow to “act with utmost restraint to de-escalate” tensions in the Black Sea.
Speaking via their foreign affairs spokeswoman Maja Kocijanic, they also demanded that Russia end their blockade and “restore freedom of passage” through the Kerch Strait so Ukrainian vessels can transit to ports in the Sea of Azov.
European Council President Donald Tusk said Europe would "stay united in support of Ukraine," echoing the Ukrainian president's demands to release the detained sailors and return the vessels.
British Foreign Secretary Jeremy Hunt joined European condemnation, saying "once again, we see Russian contempt for international norms and Ukrainian sovreignty."