Germany concerned about escalation of tensions between Russia and Ukraine
"The developments are troubling," he tweeted.
"A Russian blockade of the passage to the Sea of Azov is unacceptable. It is important that the blockade be lifted. We call on both sides to deescalate."
German government spokesman Steffen Seibert said later that Berlin was following the reports with "great concern" and called for "restraint" to prevent further escalation.
"We will closely monitor developments," he told reporters, adding that the German government was in contact with Kiev and Moscow.
He said the use of military force by Russia was "unjustified based on the facts we have seen to date" and called "both sides to quickly resume dialogue".
The confrontation at sea has raised fears of a wider military escalation and the UN Security Council was to hold an emergency session later Monday to consider a response.
(With inputs from agencies.)
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