The move comes after tensions escalated over Russia's capture of Ukranian ships accusing its neighbour of illegally entering Russian waters off the coast of Crimea in the Sea of Azov. Ukraine's military is on high alert over the incident.
The Ukranian president's martial law decree, however, needs parliamentary approval.
Meanwhile, Britain condemned Russia's "act of aggression" against Ukrainian ships in the Kerch Strait off the coast of Crimea.
Russia on its part ignored Western calls to release three Ukrainian naval ships and their crews it captured near Crimea on Sunday. The Russian foreign ministry blamed Ukraine for the current crisis.
"It's obvious that this painstakingly thought-through and planned provocation was aimed at igniting another source of tension in the region in order to create a pretext to ramp up sanctions against Russia," the Russian foreign ministry said in a statement.
"We'd like to warn the Ukrainian side that the policy of provoking a conflict with Russia in the area of the Sea of Azov and the Black Sea, which has been pursued by Kiev in coordination with the United States and the European Union, is fraught with serious consequences," it added.
The United States said Russia was involved in "outlaw actions" in the Sea of Azov.
"The United States would welcome a normal relationship with Russia. But outlaw actions like this one continue to make that impossible," US ambassador Nikki Haley told the UN Security Council.
Ukraine and Russia have been in a state of uneasy peace after the Russian military annexed Crimea in 2014.
Russia's Interfax news agency reported that 24 Ukrainian sailors were detained and at least three were wounded in the clash with the Russians over the weekend.
The Sea of Azov has become the latest bone of contention between the two countries after Russia built a bridge connecting Russia's Krasnodar to Crimea. Ukrainian vessels have to tiptoe across Russian ships from the Black Sea through the bottleneck of the Kerch Strait and into the Azov Sea to reach Ukrainian ports.
A bilateral treaty signed in 2003 had given both Russia and Ukraine the right to access the Sea of Azov.
Britain condemned Russia's "act of aggression" against Ukrainian ships in the Kerch Strait off the coast of Crimea.