Ukrainian President Petro Poroshenko on Monday accused Russia of taking its conflict with Kiev to another level after Moscow seized three Ukrainian ships near Crimea at the weekend.
"After the attack on Ukrainian military vessels, (Russia) entered a new phase of aggression," Poroshenko said in a televised address to the nation.
He said the incident showed "the arrogant and open participation of regular units of Russian troops" in the conflict with Ukraine.
This "changes the situation," he said, because Russia has previously denied the involvement of its regular troops in the conflict in Ukraine.
Poroshenko addressed the nation ahead of a parliamentary vote on his request to impose martial law in the country.
"We have to reinforce our defence immediately in order to be able to quickly react in case of an invasion," he said.
The Ukrainian leader said he would ask parliament to introduce martial law for 30 days, after initially saying it would last 60 days.
Poroshenko said he reduced the period so that "it will not overlap for a single day with the start of the election campaign" next year.
Critics earlier accused Poroshenko of wanting to introduce martial law to delay a presidential election expected to be held in March.