Secretary of State Mike Pompeo pledged the United States' support of Ukraine one day after a naval skirmish between Ukraine and Russia put the countries closer to a full-scale conflict.

A Russian ship is reported to have opened fire on a Ukrainian vessel, while another was caught on video ramming a Ukrainian tug boat off the Crimean coast Sunday.

Russia also seized three Ukrainian ships.

"The United States expresses its deep concern over the incident in the Black Sea that occurred Nov. 25 as Ukrainian vessels attempted to transit through the Kerch Strait," Pompeo said in a statement Monday evening. "Reports that Russian vessels rammed and fired on the Ukrainian ships, injuring Ukrainian crewmen, before seizing three vessels, represent a dangerous escalation and a violation of international law.

"The United States condemns this aggressive Russian action. We call on Russia to return to Ukraine its vessels and detained crew members, and to respect Ukraine's sovereignty and territorial integrity within its internationally recognized borders, extending to its territorial waters."

Pompeo said the U.S. is asking both nations involved to "exercise restraint" in resolving the matter.

"The United States supports Ukraine's sovereignty and territorial integrity within its internationally recognized borders, extending to its territorial waters, as well as the right of its vessels to traverse international waters," Pompeo said. "As stated in our Crimea Declaration, the United States rejects Russia's attempted annexation of Crimea."

Russia invaded the Crimean peninsula, part of Ukraine, in 2014.

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