Ukraine said its soldiers were injured after a Russian border guard vessel allegedly rammed into one of its tugboats in the Kerch Strait, a narrow body of water that gives access to the Sea of Azov and is used by both nations.
Over the weekend, Poroshenko urged Germany and other Western allies to boost their naval presence in the Black Sea to help deter Russian Federation from further aggression.
Merkel criticized that Russian move, speaking to reporters Saturday on the sidelines of the Group of 20 summit in Buenos Aires after holding a bilateral meeting with Russian President Vladimir Putin.
The separatist conflict in eastern regions forming Ukraine's industrial heartland has taken a toll on the national economy, reducing the cargo flow through the Ukrainian ports of Mariupol and Berdyansk on the Sea of Azov.
Trump on Thursday announced on Twitter that "it would be best for all parties concerned" if he and Putin did not meet as planned because Ukrainian ships and sailors recently seized in the Sea of Azov had not been released by Russian Federation.
In the November 25 incident, three Ukrainian naval vessels were heading from the Black Sea into the Sea of Azov when they were blocked by the Russian coast guard near the Kerch Strait between Russia's mainland and Crimea.
The martial law order included a ban on entry to Ukraine of all Russian males aged 16 to 60, a move Poroshenko said is created to prevent undercover Russian military units from infiltrating the country.
Citing the imminent threat of a Russian offensive, Poroshenko has convinced the Ukrainian parliament to implement 30 days of martial law.
"Ukraine is taking its own steps in response to the threat of a large-scale Russian invasion", the president said.
"In the waters of the Black Sea, Sea of Azov, and Aegean Sea, more than 80 ships and 8 submarines are on patrol - including 23 combat vessels and 6 submarines", he declared.
Poroshenko on Saturday repeated allegations that Russia had amassed a large military force in eastern Ukraine, which is controlled by pro-Russian separatists, and in Crimea, which Russia annexed from Ukraine in 2014. An unsteady cease-fire has been in place there since 2015.
Ukrainian President Petro Poroshenko said Sunday that Orthodox Christians would be free to remain part of the Russian branch of the church following the establishment of an autocephalous Ukrainian Orthodox Church.
Putin put blame on Ukraine for the latest dispute between the two countries, adding that he considered it "too early" to return the ships and sailors.