Ukraine has banned Russian adult men aged 16-60 years from entering Ukraine, the head of the border service Petro Tsygykal said in a televised meeting on Friday.

“Today, the entry of foreigners is limited - primarily citizens of the Russian Federation - non-admission of citizens of the Russian Federation aged from 16 to 60, male,” Mr Tsygykal said.

Ukraine imposed martial law this week, citing fears that Russia was planning a full scale invasion after Russian vessels fired on and captured Ukrainian ships last weekend.

As tensions rose, US president Donald Trump cancelled highly anticipated talks with Russian counterpart Vladimir Putin due to his failure to free three Ukrainian naval boats and 24 crewmen that his forces captured in the Sea of Azov.

“Based on the fact that the ships and sailors have not been returned to Ukraine from Russia, I have decided it would be best for all parties concerned to cancel my previously scheduled meeting,” at this week’s G20 summit in Argentina, Mr Trump tweeted on Thursday.

The announcement came about one hour after Mr Trump had said outside the White House: “I think it’s a very good time to have the meeting. I’m getting a full report on the plane as to what happened with respect to [Ukraine] and that will determine whether I will be meeting.”

The surprise cancellation will be welcomed in Kiev, which urged Nato to send warships to the Sea of Azov following Russia’s capture of the boats and sailors and subsequent imposition of a de facto blockade on Ukrainian ports in the area.

“Germany is one of our closest allies, and we hope that Nato states are now prepared to send naval ships to the Sea of Azov to support Ukraine and provide security,” Ukrainian president Petro Poroshenko told Germany’s Bild newspaper.