The UN has the power and the ships to Evacuate all citizens from the Maripoul Steel plant and any other civilians who wish to leave. The Sea of Azov is connected to the Black Sea.

if Russian UN officials block this effort they should be immediately  arrested and sent to the Hauge  for Crimes against Humanity.  

https://www.marketwatch.com/story/ukraine-russians-try-to-storm-mariupol-plant-strike-odesa-01650740445?siteid=yhoof2&yptr=yahoo