WASHINGTON (Reuters) - Russia’s seizure of three Ukrainian navy vessels represents “a dangerous escalation and a violation of international law,” U.S. Secretary of State Mike Pompeo said on Monday in a statement that called for restraint from both countries.

Ukraine imposed martial law on Monday for 30 days after Russia seized two small Ukrainian artillery vessels and a tug boat and detained their crews over the weekend. The top U.S. diplomat condemned Russia’s actions and called for Moscow to return the vessels.