Russia on Monday ignored Western calls to release three Ukrainian naval ships and their crews it fired on and captured near Crimea over the weekend and accused Kiev of plotting with its Western allies to provoke a conflict.
Ukraine in turn accused Russia of military aggression and put its armed forces on full combat alert, saying it reserved the right to defend itself. Ukrainian lawmakers were due to decide later on Monday whether to approve President Petro Poroshenko's call to impose martial law in Ukraine for two months.
With relations still raw after Russia's 2014 annexation of Crimea from Ukraine and its backing for a pro-Moscow insurgency in eastern Ukraine, the crisis risks pushing the two countries towards a wider conflict and there were early signs it was reigniting Western calls for more sanctions on Moscow.