Mariupol is where the Ukrainian army is digging in after getting routed Novoazovsk.

Mariupol is the main Ukrainian port in the south eastern part of the country and has a population of 500,000.

It would be quite a provocation to attack Mariupol on the same day as the NATO Summit.

Update: There is now talk of fighting North of the city and images of vehicles in flames are beginning to appear on twitter.

Mariupol fighting

Rumored to be a Russian tank