Ukrainian soldiers guard a checkpoint at the Mariupol Port | Martyn Aim/Getty Images Ukraine bans Russian men from entering country The Ukrainian border service has ‘switched to the first level of full combat readiness,’ says its head Petro Tsygykal.

Ukraine banned Russian men aged between 16 and 60 years old from passing checkpoints into the country, President Petro Poroshenko said Friday.

The measures were introduced after Ukraine implemented martial law earlier this week in response to an incident in which Russian forces opened fire on three Ukrainian naval ships off the coast of annexed Crimea. They are aimed at preventing a Russian land invasion, according to the president.

The Ukrainian border service has "switched to the first level of full combat readiness," its head Petro Tsygykal told the president, according to a statement.

"Reinforced control measures are being implemented" at checkpoints, Tsygykal said. "Citizens of the Russian Federation aged from 16 to 60 years old are not allowed to pass, male persons."

Russian men who wish to enter Ukraine can apply for a permit on “humanitarian” grounds such as to attend funerals, Tsygykal said.

Non-Russian foreigners wishing to cross into Ukraine at checkpoints will be subject to "enhanced interviewing," border service spokesman Andrei Demchenko told broadcaster 112 Ukraine.

Russia’s foreign ministry spokeswoman Maria Zakharova said at a briefing Friday that "it's simply frightening" for Moscow to consider "mirroring" Ukraine's measures, TASS reported.

As part of the measures introduced under martial law, the country has set up a task force including representatives of the defense ministry, the armed forces, the chief directorate of intelligence and the state transport service. The group is tasked with addressing potential supply problems resulting from an effective Russian blockade of two of Ukraine's Black Sea ports to ensure continued transport of weapons and equipment for the defense ministry.

"Today we are working to replenish all kinds of stocks," Defense Minister Stepan Poltorak said, according to a statement.

Ukraine is also deploying more units along its border with Russia and to the administrative boundary of the annexed Crimea, reinforcing them with more weapons and armored vehicles.

Poroshenko said in a series of tweets that the measures are designed to prevent Russia from forming units of "private armies" in Ukraine to carry out a land invasion.

Ukraine implemented martial law for 30 days in 10 border areas, with the measures to be in place until December 26.