An industrial hub of half a million people, Mariupol sits on the fringe of Ukraine's year-long insurgency, which has killed more than 6000 people and ruined Russia's ties with its Cold War foes. An attack would bury the latest Minsk truce brokered by Russia and Germany and risk more sanctions for Mr Putin's government. It could also reignite calls to arm Ukraine.

Ukraine's armoured personnel carriers en route to a front-line position east of Mariupol in March. Credit:AP

"If a new attack takes place, the Minsk agreements will be ultimately dead, and a new initiative at negotiating a lasting settlement – in whatever format – is unlikely for longer", said Joerg Forbrig, senior program director at the German Marshall Fund in Berlin. "The US and several European Union countries will see a renewed debate on lethal arms supplies."

Sitting just 25 kilometres from rebel positions, between Russia and the Crimean peninsula Mr Putin annexed last March, locals see reasons to be scared.

In Mariupol's outskirts, shelling is frequently heard from the nearby town of Shyrokyne. Military personnel flood the city, building defensive positions and bunkers and blocking approach roads with huge concrete blocks. Visitors arriving by bus must clear multiple checkpoints where searches can take hours.