"In accordance with the National Security and Defence Council's decision on imposing martial law, the chief of the General Staff and commander-in-chief of Ukraine's armed forces ordered to put the Ukrainian armed forces' units on full combat alert", Ukraine's defence ministry confirmed in a statement.
On Sunday, Russia dispatched warplanes to patrol the area after the Ukrainian navy tried to send the ships through the Kerch Strait, a waterway with strategic significance for both countries that passes under a newly built Russian bridge.
Poroshenko also said that Ukraine would stick to the Minsk Agreements, peace deals his government inked with Russia and pro-Russian separatists in southeastern Ukraine that were created to stop a war that claimed more than 10,000 lives since 2014. Ukraine, along with almost every other country in the world, refuses to recognize that claim.
Russian Federation fired on and seized three Ukrainian vessels - two gunships and a tug named as Yana Kapa, the BBC reported. The boats were towed to a nearby port.
Russia did not immediately or directly respond to the allegation, but Russian news agencies cited the FSB security service as saying it had incontrovertible proof that Ukraine had orchestrated what it called "a provocation" and would publicise its evidence soon.
Trump has not commented on the incident.
Russia's Federal Security Service (FSB) told Russian news agencies that the Ukrainian ships held their course and violated Russian territorial waters, which were temporarily closed.
Kevin Faulk of LSU involved in skirmish after Texas A&M game
Texas A&M outlasted LSU in seven overtimes in College Station as the game stretched into the early hours of Sunday morning. If the reason Kevin Faulk got into the fight is accurate, Faulk should get a raise and the Texas A&M guy should be fired.
Ukraine's ambassador to the United Kingdom said Russian special forces had captured two armoured artillery boats and a tugboat in an "act of aggression".
"We consider it as an act of aggression against our state and a very serious threat", the President said.
"Russia's provocative actions in the Black Sea and the Sea of Azov have crossed the line and become aggressive", it said. Moscow says it had not been notified in advance of the flotilla's passage and the boats ignored warnings to stop.
The incident sparked an global response and concern mainly for Ukraine over its more powerful nemesis.
In a comment posted on Facebook, Kosachev also said that Kiev's provocation in the Kerch Strait made "absolutely no sense from the military point of view - although it is rather promising idea from the political point of view: both North Atlantic Treaty Organisation and the European Union are sure to stand up for Kiev's provokers, no matter what happens".
"Nato. should send in naval ships in the Sea of Azov", the Atlantic Council's Anders Aslund also said. Moscow's military intervention in the peninsula and subsequent referendum inflamed separatist sentiment in eastern Ukraine, effectively triggering a rebel insurgency by pro-Russia forces.
Turkey stressed that as a country sharing a Black Sea coast "passage through the Kerch strait should not be blocked", and called for an avoidance of any actions likely to endanger stability and peace in the region.