Ukraine leader warns of threat of full-scale confrontation with Russia

President Petro Poroshenko told Ukrainian journalists that the incident Sunday was an "extraordinary event."

"Officially, without tearing off chevrons, without 'green men,' Russian troops in large numbers attacked the ships of the Armed Forces of Ukraine," Poroshenko said on his website, referring to soldiers during the 2014 Russian invasion and annexation of Crimea who were not wearing insignia on their green uniforms.

The leader also pointed to what he called a strong Russian military presence along the border with his country.

"And that is why I don't want anybody to think that it's a trifle. The country is under the threat of a full-scale war with the Russian Federation," Poroshenko said.

Earlier, he told CNN that US Secretary of State Mike Pompeo pledged to Ukraine "full assistance, including military assistance," after Sunday's confrontation between Ukraine and Russia.

Poroshenko told CNN's Christiane Amanpour that during a recent phone call Pompeo pledged "full coordination" on protecting Ukrainian sovereignty following the clash with Russia.

He also said the Ukrainian government was considering limiting Russians' ability to enter Ukraine during the period of martial law.

"One of the proposals to our chief of general staff and the chief of the border control is that (there) would be some limits for Russians to enter to Ukraine during this special period," Poroshenko said from Kiev, adding that a final decision would likely be made tomorrow.

Sailors detained for two months

Poroshenko's comments come after a court in Russian-controlled Crimea ordered that 12 of the Ukrainian sailors captured during clashes Sunday be detained for two months. It was a move likely to further inflame tensions between the two countries.

A judge in the Crimean town of Simferopol said Tuesday that they have been detained pending trial, charged with trespassing the Russian border, Russian state media reported.

The sailors, including the commander of a small Ukrainian gunboat, will be held under formal arrest in a detention center in Simferopol until January 25, the judge ruled.

Court proceedings will resume on Wednesday for the remaining sailors.

The decision comes after a confrontation between the two countries around a waterway linking the Azov Sea and the Black Sea, when Russia fired on and seized three Ukrainian navy ships and detained 24 Ukrainian sailors.

Ukraine and Russia have offered conflicting accounts of the incident, each accusing the other of violating the laws of the sea.

Ukraine said two of its small gunboats and one tugboat were attacked by Russian naval forces Sunday after entering the Kerch Strait en route to the city of Mariupol.

Video of the incident released by Ukrainian officials appeared to show a Russian ship ramming the Ukrainian tugboat.

The incident provoked fury in Kiev, while Russia accused Ukraine of acting as the aggressor and called the incident a "dangerous provocation" by Kiev.

The Kerch Strait -- a shallow, narrow stretch of water just two to three miles wide at one point -- connects the Azov Sea with the Black Sea and runs between the Crimean Peninsula and Russia. It is an important economic lifeline for Ukraine, as it allows its ships to access the Black Sea.

It's also the closest point of access for Russia to Crimea, which Moscow annexed from Ukraine in 2014. A Russian-built bridge over the Kerch Strait was opened in May.

A 2003 agreement confirms the Azov Sea and the Kerch Strait as domestic waters of Russia and Ukraine.

Ukraine to impose martial law

On Monday, the Ukrainian Parliament voted to introduce martial law from Wednesday in select regions for 30 days, said the Ukrainian news agency Ukrinform, endorsing an emergency proposal introduced by Ukrainian President Petro Poroshenko.

In a statement on Twitter, Poroshenko said martial law would be introduced only in regions bordering Russia, or adjacent to regions where Russian troops are posted. That includes regions bordering the Azov and Black Seas and a portion of the border with Moldova's Transnistria region, where Russian troops are stationed.

Kremlin spokesperson Dmitry Peskov said Ukraine's decision to introduce martial law had a "barely camouflaged" political agenda, although he added that the move was an internal matter for Kiev.

"This is Ukraine's internal business," Peskov said before adding, "Against the background of the electoral processes, of course, such decision to impose martial law has a very specific, barely camouflaged overtone."

Ukraine is slated to have presidential elections in March, after the period of martial law expires.

Russia and Ukraine have been locked in long-simmering conflict since the annexation of Crimea by Russia in 2014, and a war with Russian-backed separatists in the country's east has claimed more than 10,000 lives.

West criticizes Russian aggression

Sunday's incident has sparked international condemnation over Russia's use of force.

During an emergency UN Security Council meeting to defuse rising tensions after the confrontation, US Ambassador Nikki Haley called the incident "yet another reckless Russian escalation" and demanded that the Kremlin release the sailors.

"Impeding Ukraine's lawful transit through the Kerch Strait is a violation under international law. It is an arrogant act that the international community must condemn and will never accept," Haley said Monday.

European member states of the UN Security Council -- including France, the Netherlands, Poland, Sweden and the UK -- also issued a statement calling on Russia to restore freedom of passage at the Kerch Strait.