Ukraine's foreign minister on Thursday called for a speedy international response to Russia's "aggression" after it seized three Ukrainian navy ships and 24 sailors in the Sea of Azov.
At a meeting of the Organisation for Security and Co-operation in Europe (OSCE) in Milan, Pavlo Klimkin said this was the fifth consecutive OSCE ministerial meeting to be dominated by Russia's "destabilising" behaviour in Europe.
"It's a major threat to European security," the minister said, in front of Russia's top diplomat who was also in attendance.
"This year, Russia did not pull back but extended its aggressive course of action," Klimkin added.
The confrontation at sea on November 25 was the first open military incident between Kiev and Moscow since 2014 when Russia annexed the Crimean peninsula and a conflict erupted in eastern Ukraine between pro-Russia separatists and government forces, which has claimed more than 10,000 lives.
Klimkin said Ukraine would return to the United Nations to demand a "prompt response to this act of aggression".
"Declarations are not enough, there must be action."
Klimkin called for the "immediate release and safe return to Ukraine" of the sailors and the ships, a demand also made on Thursday by EU foreign policy chief Federica Mogherini and US Assistant Secretary of State for European and Eurasian Affairs Wess Mitchell.
Mitchell denounced Moscow's "dangerous escalation", saying it was an attempt to "negate the integrity of Ukraine".
"Russia has violated international law," he said, adding that the United States would never recognise the annexation of Crimea.
"It is time for Russia to rethink its approach, to respect international laws, to fulfil its commitments as an OSCE state and be a member of the community of civilised nations."
Mogherini called for the "greatest moderation to proceed toward a de-escalation" of the conflict.
The sailors have been put before a court in Simferopol, the main city in Russian-annexed Crimea, and ordered to be held in pre-trial detention for two months.
The first sign of detente appeared on Tuesday when Ukraine announced the "partial" lifting of the blockade by Russia in order to access some ports in the Sea of Azov.
Moscow has defended its actions, maintaining that the Ukrainian ships had entered Russian territorial waters and refused to respond to requests to stop from Russia's patrol boats.
Russian Foreign Minister Sergei Lavrov meanwhile denounced "Ukraine's behaviour" at the meeting and its support from the West.
He blasted "the reckless enlargement of NATO... the deployment of US anti-aircraft defence systems in Europe, as well as illegitimate sanctions (imposed) under false pretexts," referring to sanctions imposed by the United States and the European Union after the annexation of Crimea.