Ukraine has blocked Russian men between the ages of 16 and 60 from entering the country as a feud with Moscow over a naval incident continues to escalate.

The ban, which was announced Friday, will last 30 days and run alongside a declaration of martial law implemented this week, The Associated Press reports.

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Tensions between Ukraine and Russia were reignited this month after Russia seized three Ukrainian ships and detained their sailors in the Kerch Strait, which links the Black Sea and the Sea of Azov.

Ukraine has said the sailors are prisoners of war who were captured in international waters, while Moscow argues that the ships violated Russia’s borders.

Ukrainian President Petro Poroshenko told security officials the ban announced Friday will prevent Russian servicemen disguised as civilians from sneaking into the country, according to the AP.

Several Russian troops were able to sneak into Eastern Ukraine where the Ukrainian army is in an ongoing battle with Russian-backed separatists.

Kiev adopted a measure Monday to impose martial law for 30 days, a measure Ukraine refrained from taking even in 2014 after Russia annexed the Crimean peninsula in 2014, which Ukraine claims as its own territory.

The annexation inflamed tensions between the two countries and further deteriorated Moscow’s relationships with the international community. The U.S. and most other nations have not recognized Moscow’s annexation of Crimea as legitimate and have imposed sanctions on Russia.

Tensions were further exacerbated after Russian state television aired interviews with some of the sailors, which Ukraine condemned.

The U.S. and its allies issued a strong rebuke of Russia’s actions at the United Nations this week.

“We strongly support Ukraine’s sovereignty and territorial integrity within its internationally recognized borders, extending to its territorial waters. We express our deep concern over the incident, which represent a dangerous escalation and violation of international law,” U.S. Ambassador to the U.N. Nikki Haley Nimrata (Nikki) Haley'The soul' versus 'law and order' Author Ryan Girdusky: RNC worked best when highlighting 'regular people' as opposed to 'standard Republicans' GOP lobbyists pleasantly surprised by Republican convention MORE said.