Ukraine on Monday imposed martial law for 30 days after a weekend naval confrontation off the disputed Crimean Peninsula in which Russia fired on and seized three Ukrainian vessels amid renewed tensions between the neighbours.

Western leaders and diplomats urged both sides to de-escalate the conflict. The U.S. blamed Russia for what it called "unlawful conduct" over Sunday's incident in the Black Sea.

The two neighbours have been locked in a tense tug-of-war since Russia's 2014 annexation of Crimea, but the incident late Sunday in which Russian coast guard ships fired on Ukrainian navy vessels near the Kerch Strait directly pitted the two militaries, placing them on the verge of an open conflict.

The Ukrainian navy said six of its seamen were wounded when the Russian coast guard opened fire on three Ukrainian ships near the Kerch Strait and then seized them. Russia said three Ukrainian sailors were slightly injured and given medical assistance.

After a five-hour debate, parliament overwhelmingly approved Ukrainian President Petro Poroshenko's proposal to institute martial law for 30 days starting Wednesday in Ukrainian regions bordering Russia, Belarus and Moldova's pro-Moscow breakaway republic of Trans-Dniester.

The 10 locations chosen were ones that Poroshenko identified as potentially in the front line of any Russian attack.

The seized Ukrainian ships anchored in a port of Kerch. (Pavel Rebrov/Reuters)

Poroshenko said it was necessary because of intelligence about "a highly serious threat of a ground operation against Ukraine." He did not elaborate.

"Martial law doesn't mean declaring a war," he said. "It is introduced with the sole purpose of boosting Ukraine's defence in the light of a growing aggression from Russia."

The new emergency measures adopted by Ukrainian lawmakers will include a partial mobilization and strengthening the country's air defence. They also include a plethora of vaguely worded steps such as the "strengthening" of anti-terrorism measures and "information security."

Poroshenko has pledged the new measures would respect the rights of Ukrainian citizens.

Timing scrutinized

Poroshenko's critics, meanwhile, reacted to the call for martial law with suspicion, wondering why Sunday's incident merited such a response.

Poroshenko's approval ratings have been plunging, and there were concerns that he would postpone a presidential election scheduled for March.

Just before the parliament met to vote, Poroshenko sought to allay those fears by releasing a statement revising his original martial law proposal from 60 days to just 30 days, in order to "do away with the pretexts for political speculation."

Russian fighter jets fly over a bridge connecting the Russian mainland with the Crimean Peninsula, after three Ukrainian navy vessels were stopped by Russia from entering the Sea of Azov via the Kerch Strait in the Black Sea on Sunday. (Pavel Rebrov/Reuters)

Oksana Syroid, a deputy speaker of parliament, noted that martial law was not introduced in 2014 or 2015 despite large-scale fighting in the east. A state of emergency "would present a wonderful chance to manipulate the presidential elections," she said.

NATO Secretary-General Jens Stoltenberg said Poroshenko assured him that martial law would not have a negative impact on the election.

Poroshenko insisted it was necessary because what happened in the Kerch Strait between Crimea and the Russian mainland "was no accident," adding that "this was not the culmination of it yet."

While a 2003 treaty designates the Kerch Strait and Sea of Azov as shared territorial waters, Russia has sought to assert greater control over the passage since the annexation.

Ukrainian Foreign Minister Pavlo Klimkin tweeted that the weekend dispute was not an accident and that Russia had engaged in "deliberately planned hostilities," while Russian Foreign Minister Sergey Lavrov blamed Kyiv for what he described as a "provocation," adding that "Ukraine had undoubtedly hoped to get additional benefits from the situation, expecting the U.S. and Europe to blindly take the provocateurs' side."

Klimkin told reporters in Kiev that the government is in talks with the Red Cross to make sure the captive seamen are treated as prisoners of war. Kremlin spokesperson Dmitry Peskov did not say whether Moscow considers them prisoners of war.

Trump equivocates

At a UN Security Council meeting, U.S. Ambassador Nikki Haley urged Russia to "immediately cease its unlawful conduct" in the Black Sea.

In his first public remarks since the confrontation, U.S. President Donald Trump did not specifically call out Russia's behaviour.

"We do not like what's happening, either way, we don't like what's happening and hopefully it will get straightened out," Trump said.

But Russia called Ukraine's actions "dangerous." Dmitry Polyanskiy, Russia's first deputy permanent representative to the United Nations, told the Security Council the incident was another example of Ukrainian leaders trying to provoke Russia for political purposes.

The European Union and NATO called for restraint from both sides. NATO said Stoltenberg expressed the U.S.-led military alliance's "full support for Ukraine's territorial integrity and sovereignty, including its full navigational rights in its territorial waters under international law."

In a statement, Canadian Foreign Affairs Minister Chrystia Freeland said her country "strongly condemns Russia's actions" and called on Moscow to release the captured vessels.

Canada condemns Russian aggression towards <a href="https://twitter.com/hashtag/Ukraine?src=hash&ref_src=twsrc%5Etfw">#Ukraine</a> in the <a href="https://twitter.com/hashtag/KerchStrait?src=hash&ref_src=twsrc%5Etfw">#KerchStrait</a>. We call on <a href="https://twitter.com/hashtag/Russia?src=hash&ref_src=twsrc%5Etfw">#Russia</a> to immediately de-escalate, release the captured vessels, and allow for freedom of passage. Canada is unwavering in its support for Ukraine’s sovereignty. —@cafreeland

"The Government of Canada is unequivocal in its support for Ukraine and in its condemnation of Russia's illegal invasion and annexation of Crimea," the statement said. "Canada will always be a steadfast partner of the people of Ukraine, and we will continue to work with our allies to hold Russia to account for its unacceptable behaviour."