Ukraine has accused Russia of military aggression and puts its armed forces on full combat alert after Russia on Sunday fired on and then captured the three ships and 24-strong crew in the Kerch Strait, which links the Black Sea with the Azov Sea.
Kiev has urged its Western allies to consider further sanctions against Moscow. Russia in its turn has accused Ukraine of plotting with the West to provoke a crisis.
"We saw that Russia used military force against Ukraine in an open and direct way," Stoltenberg told a news conference after emergency talks of the Western military alliance held at the request of Ukraine, which is not a NATO member.
"All allies expressed full support for Ukraine's territorial integrity and sovereignty," he added. "There is no justification for the use of military force against Ukrainian ships and naval personnel so we call on Russia to release immediately the Ukrainian sailors and ships it seized yesterday."
Stoltenberg said Russia should also allow all commercial vessels full access to Ukrainian ports.
He cited economic sanctions already imposed by the West on Russia over its 2014 annexation of Ukraine's Crimea peninsula and its support for separatist rebels battling Kiev's forces in eastern Ukraine.
"We are following and monitoring the situation very closely and we are constantly assessing what more we can do because Russia has to understand that its actions have consequences."
"This is escalating the situation in the region and it confirms a pattern of behaviour which we have seen over several years...," he added. (Reporting by Gabriela Baczynska and Alissa de Carbonnel Editing by Gareth Jones)
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