Ukraine will restrict the entry of Russian men aged between 16 and 60, in an effort to prevent Russia from forming "private armies" on its soil, President Petro Poroshenko said on Friday.

Poroshenko cited fears of an invasion similar to Russia’s seizure and annexation of the Black Sea peninsula of Crimea in 2014 as the logic behind his latest security measures.

The ban follows Kiev’s imposition of 30 days of martial law in a handful of provinces. Tensions have soared between the two former Soviet Republics after Russian boats had attacked and seized three Ukrainian navy vessels and 24 sailors in the Kerch Strait last weekend.

A court in Crimea ordered all 24 sailors to be detained for two months earlier this week pending trial. On Thursday, a number of the sailors were transferred to Moscow, according to a lawyer for two of the sailors. CNN reached out to the detention centre but did not receive a response.

On Friday, Russian Foreign Ministry spokesperson Maria Zakharova said the entry ban on Russian men showed Kiev’s “dysfunction” but stopped short of calling for a tit-for-tat retaliation.

“The somersaults that we see in Kiev, and in Ukraine in general, show the complete dysfunction of the state, of the state as an entity, a consequence of the wild – and it’s impossible not to say premeditated – movements of the country’s leadership,” she added.

Earlier this week, Poroshenko said Russia appeared to be building its military presence along the border, sparking further concerns that the long-simmering conflict is entering a new phase.

“The country is under the threat of a full-scale war with the Russian Federation,” Poroshenko said.

There have also been reports of Russian missiles being moved in the aftermath of the stand-off.

The Russian military will deploy additional S-400 air-defence systems to Crimea, Russian state news agency RIA-Novosti reported Tuesday, citing the head of the press service of Russia’s Southern Military District, Vadim Astafyev.

The clash and subsequent aftermath has prompted fears that Russia may seek to control all entry and movement in the Azov Sea.

Russia’s use of force against Ukraine sparked widespread condemnation, and the crisis is likely to loom large over the G20 summit in Argentina, which began on Friday.

On Thursday, US President Donald Trump abruptly called off a bilateral meeting with Russian leader Vladimir Putin due to take place on the sidelines of the two-day event.

Trump blamed the cancellation on Russia’s refusal to release the Ukrainian navy ships and sailors seized during last week’s incident.

While the decision was applauded in Kiev, some tied the cancellation to the Trump’s domestic political problems. Hours before Trump pulled the plug on the sit-down with Putin, new allegations surfaced involving his former longtime attorney Michael Cohen, once again prompting questions about Trump’s financial ties to Russia.

The Kremlin said it regretted the US administration’s decision to cancel the meeting but left the door open to reschedule the appointment.

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