Russian president Vladimir Putin has expressed “serious concern” over Ukraine’s decision to impose martial law, the Kremlin said on Tuesday, as the simmering confrontation between Moscow and Kiev sparked a new global crisis.

In a phone conversation with Chancellor Angela Merkel, Putin also said he hoped the German leader could intervene to rein in Kiev. Putin “expressed a serious concern over Kiev’s decision to put its armed forces on alert and to introduce martial law,” the Kremlin said in a statement following the call.

He also said he hoped “Berlin could influence the Ukrainian authorities to dissuade them from further reckless acts,” it added.

The political efforts came after Russia fired on and seized three Ukrainian vessels and their crews in the Kerch strait separating Crimea from the Russian mainland. Ukrainian MPs responded by voting to impose martial law.

Six Ukrainians were reported to be injured, one of them critically, in the clash at the mouth of the Sea of Azov, where Russia has been building up its naval presence and seeking to restrict Ukrainian access since completing a bridge across the strait in May. The Ukrainian government released video footage of one of its ships being rammed by a Russian vessel.

The incident sparked an emergency debate at the UN security council, where the Russian and Ukrainian ambassadors accused each other’s governments of seeking to trigger a conflict to deflect from their own domestic unpopularity.

The Ukrainian ambassador to the UN, Volodymyr Yelchenko, said the Russian naval authorities had been notified that the three Ukrainian vessels – two cutters and a tugboat – wished to pass through the strait, and had been waiting to hear confirmation on Sunday morning when the vessels were attacked.

“All hell broke loose,” Yelchenko said, adding that the Ukrainian tugboat had been rammed by a Russian coastguard vessel and the two gunships had been blocked from coming to its assistance. “All this time, the Russian vessels were trying to provoke Ukrainian seamen to open fire.”

He said the Ukrainian cutters had then been buzzed by Russian attack helicopters flying at 50 metres and displaying their missiles. The cutters had then turned around and headed back into the Black Sea, the ambassador said. As they were steaming away, they were pursued by Russian ships, which opened fire at 8.30pm. Russian special forces then seized the ships.

The Russian ambassador, Dmitry Polyanskiy, claimed the Ukrainian ships had entered Russian territorial waters and portrayed the incident as a long-planned provocation by the Ukrainian president, Petro Poroshenko, and his western backers to distract from Poroshenko’s unpopularity in the run-up to elections scheduled for next March. Ukraine’s declaration of martial law “is about cancelling the elections”, he said.

Polyanskiy said the Ukrainian ships had “illegally crossed Russia’s border” and the “responsibility lies with those who gave the illegal order” for the ships to cross the Kerch strait.

Since the completion of the bridge over the Kerch strait, Moscow has demanded that Ukrainian ships not only give notice of their intention to transit the strait but request permission, a change that Kiev has rejected. According to western diplomats, the dispatch of the three ships was intended to assert freedom of navigation and also to reinforce a very small Ukrainian naval presence in the Sea of Azov.

The US ambassador to the UN, Nikki Haley, said of Russia’s actions: “This is no way for a law-abiding, civilised nation to act. 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.”

Later, the US secretary of state, Mike Pompeo, accused Moscow of “a dangerous escalation and a violation of international law”, adding: “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 recognised borders, extending to its territorial waters.”

Donald Trump, when asked what he thought about the crisis, said: “Not good. Not happy about it all.” The US president did not apportion blame – he has sought to avoid criticism of the Kremlin throughout his two years in office – but said the US had “let our position be known”.

Poroshenko signed a bill to impose martial law for 60 days, allowing troops to be mobilised, air defences to be readied and anti-terrorism measures to be reinforced. Parliament voted late on Monday to approve the measure.

The Ukrainian foreign minister, Pavlo Klimkin, said the captured sailors should be treated as prisoners of war. “These were planned acts of aggression,” he said. “We will demand [at the security council] the immediate release of our sailors and the liberation of our ships.”

In a statement accusing Ukraine of deliberately provoking the incident to provide a pretext for further anti-Russian sanctions, the foreign ministry in Moscow said Russia would respond harshly to any attempts to undermine its sovereignty and security.

“The Russian side acted strictly within both domestic and international law,” said the Kremlin spokesman, Dmitry Peskov, during a daily press briefing, adding that Moscow had opened a criminal case “in connection to the violation of Russia’s borders”. He gave no details on the fate of the Ukrainian sailors held by Moscow.

Dozens of far-right activists protested against the Russian activity in front of Poroshenko’s office in Kiev and outside the Russian consulate in the western Ukrainian city of Lviv on Monday.

Facebook Twitter Pinterest Far-right activists protest in front of the presidential administration headquarters in Kiev on Monday. Photograph: Gleb Garanich/Reuters

Nato said its ambassadors and Ukraine’s envoy would hold emergency talks in Brussels on Monday at Poroshenko’s request after he had spoken to the Nato secretary general, Jens Stoltenberg.

It called for “restraint and de-escalation” and demanded that Russia “ensure unhindered access to Ukrainian ports in the Sea of Azov in accordance with international law”.

Russian-Ukrainian relations have been severely strained since Russia’s 2014 annexation of the Crimean peninsula and its support for a pro-Moscow insurgency in eastern Ukraine. In May the Russian president, Vladimir Putin, opened the £2.9bn bridge over the Kerch strait linking the Russian mainland and the peninsula.

Facebook Twitter Pinterest The bridge connecting the Russian mainland with the Crimean peninsula at the Kerch strait. Photograph: Pavel Rebrov/Reuters

A spokesman for the British prime minister, Theresa May, said the UK condemned Russia’s “act of aggression”, while the foreign secretary, Jeremy Hunt, tweeted that the incident showed Russian “contempt for international norms and Ukrainian sovereignty”.

The EU said it expected Russia to restore freedom of passage via the Kerch strait and urged both sides to “act with the utmost restraint to de-escalate the situation”.

Observers said the incident could lead to a fresh round of sanctions by the US and Europe against Moscow. “Western governments will side with Ukraine against Russia over the incident … making new sanctions against Russia likely,” said the political risk consultancy Eurasia Group.