BERLIN (Reuters) - A Russian blockade of the Sea of Azov is unacceptable and developments there are alarming, German Foreign Minister Heiko Maas said on Twitter on Monday, urging both sides to de-escalate the conflict.

“Developments in Ukraine are worrying,” Maas said.

Russia’s FSB security service said early on Monday its border patrol boats had seized two small Ukrainian armored artillery vessels and a tug boat after opening fire on them and wounding several sailors. It is the most serious clash between Moscow and Kiev in years.