Ukraine on Monday imposed martial law in parts of the country as President Petro Poroshenko warned of the "extremely serious" threat of Russian invasion.

Poroshenko said in a televised address that the move was necessary after Russian ships attacked Ukrainian vessels off the coast of Crimea on Sunday — a major escalation of tensions in the region.

The country's parliament granted him emergency powers in certain areas of the country it considered most vulnerable to attack, and suspended elections for 30 days.

Several countries firmly denounced Russia's use of force in the stand-off, but President Trump refused to condemn Russian aggression.

Russia has accused Kiev of plotting a "well-thought-out provocation" in order to justify ramping up sanctions against them.

Ukraine on Monday imposed martial law in parts of the country as President Petro Poroshenko warned of the "extremely serious" threat of attack by Russian forces.

Poroshenko on Monday said in a televised address that the move was necessary after Russian ships attacked Ukrainian vessels off the coast of Crimea on Sunday.

"Russia has been waging a hybrid war against our country for a fifth year. But with an attack on Ukrainian military boats it moved to a new stage of aggression," Poroshenko said.

Ukraine says Russia opened fire on its navy and seized three of its vessels, injuring at least six of its servicemen. Russia claims the ships entered Russian waters illegally, and gave them warning to turn back.

WATCH: Shocking video shows the exact moment a suspected Russian ship rams a Ukrainian boat during a tense naval clash

Poroshenko said in his video address that martial law was necessary as intelligence services had evidence that Russia was preparing for a massive incursion.

"Here on several pages is a detailed description of all the forces of the enemy located at a distance of literally several dozens of kilometers from our border. Ready at any moment for an immediate invasion of Ukraine," he said.

The country's parliament granted him emergency powers in areas of the country most vulnerable to attack, and suspended elections for 30 days.

Critics alleged that Poroshenko's request for martial law was an attempt to postpone elections scheduled for next year, though lawmakers confirmed the polls would take place as scheduled.

President Donald Trump said he was working with EU leaders to assess the situation, though he refused to condemn Russian aggression. Secretary of State Mike Pompeo called the incident "a dangerous escalation" and a violation of international law, and called on both countries to exercise caution.

Several countries, including Britain, France, Poland, Denmark, and Canada, denounced Russia's use of force.

Russia has been steadily increasing its control around the Crimean peninsula, which it annexed in 2014. Sunday's stand-off came to a head after Russia used a huge tanker to block passage through the Kerch Strait — the only access point to the Sea of Azov, which is shared by both Ukraine and Russia.

The Sea of Azov has been a flash point in the conflict between the two countries. In May, Russia completed its construction of a massive 18-kilometer (11.2 mile) bridge linking the Crimea peninsula to mainland Russia.

Russia's foreign ministry accused Ukraine of "well-thought-out provocation" in order to justify ramping up sanctions against them. Russia also alleged that Kiev was working in coordination with the US and EU and warned of "serious consequences."