KIEV, Ukraine — Ukraine toughened its stance in its increasingly tense confrontation with Russia on Friday, banning entry to all Russian men of fighting age in what Ukraine’s president said was an effort to prevent the Kremlin from destabilizing the country.

The ban will last 30 days — as long as the martial law introduced this week in much of Ukraine after Russia seized three Ukrainian naval vessels and their crews.

The naval incident further escalated the tug-of-war that began in 2014 when Russia annexed Ukraine’s Crimean Peninsula and supported separatist rebels.