A senior official of the U.S. Department of State has said Russia needs to release the crews of recently captured Ukrainian boats, or there will be consequences.

"We want the Russians to absorb the message that they need to release the crews or there will be consequences and the pain will grow over time," the official said at a special briefing on December 4, 2018, on the meeting of the North Atlantic Council.

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"It marks an unmistakable escalation of the conflict there, not least because it's the first time that the Russian government has openly and unapologetically used its own forces without any attempt at claiming it was done by so-called separatists; but secondly the demonstration effect of what happened in Kerch. There are a lot of international passage – maritime passageways in the world – the Middle East, Asia. We have principled reasons to be concerned about a demonstration like – the demonstration effect like this sinking in, but also very practical and interest-based reasons to be concerned about a lot of places in the world where U.S. troops and commerce pass through, and we don't want this precedent to stick," he said.

"Today the U.S. reiterated that we condemn this Russian act of aggression, call it for what it is; that Crimea is Ukraine; and that the Russian action in Kerch is both a clear military escalation and a violation of international law and freedom of the sea."

The official listed the measures with which Washington had responded to Moscow's actions, such as sanctions against companies involved in the construction of the Kerch Strait Bridge and operating in Russian-occupied Crimea, condemnation statements by the U.S. president, the secretary of state and the U.S. ambassador to the UN, as well as the cancellation of the meeting between U.S. President Donald Trump and his Russian counterpart Vladimir Putin.

"I think the Russians have this message; but if they don't, it should be abundantly clear to them that for as long as they hold these crew members, we will continue to raise the costs. They need to release the crews, return the ships, and this is not something that we're going to turn our attention away from," he added.