Ukraine interim prime minister Arseniy Yatsenyuk put his country firmly in the Western camp Friday, signing the political provisions of a landmark association accord with EU leaders in defiance of Russia.

"This gesture symbolizes the importance both sides attach to this relationship ... and the joint will to take it further," EU president Hermann Van Rompuy said.

Ukraine was to take a step closer towards the EU on Friday after the European bloc and the U.S. put in place sanctions on Russian figures close to President Vladimir Putin as punishment for Crimea's annexation.

Moscow has said it will retaliate by issuing its own list of sanctions against senior U.S. officials but there were already signs of the harsher toll Russia was having to bear as the crisis rumbled into a new phase.

The Fitch ratings agency on Friday revised its outlook for Russia to negative from stable after the United States slapped new sanctions against Russian officials amid the Ukraine crisis.

"The revision of the outlook to negative reflects the potential impact of sanctions on Russia's economy and business environment," Fitch said in a statement.

The Ukraine crisis has brought back the fear and risk of war in Europe, European Parliament head Martin Schulz said Thursday as EU leaders met to decide their next steps.

With the 100th anniversary of the outbreak of World War I this year, who could have imagined "that war could become a genuine possibility in a country which shares a border with the European Union?," Schulz told the opening of the summit of the 28 EU leaders.

Ukraine's defense ministry on Thursday said around 20 gunmen seized a Ukrainian warship, the Ternopil, in the port of Sevastopol in Crimea.

"The ship has been taken," Vladislav Seleznyov, the defense ministry's spokesman in Crimea, said on his Facebook page, after earlier telling Agence France Presse: "The assault has begun".

Ban Ki-moon on Thursday called for U.N. and OSCE rights monitors to deploy in Ukraine and urged an "honest and constructive dialogue" between Moscow and Kiev after talks with Russia's President Vladimir Putin.

The U.N. chief said they discussed "legitimate concerns that Russia and President Putin has, particularly in terms of human rights protection and of those Russian-speaking people and Russian minorities."

U.S. President Barack Obama announced a new round of punitive measures for Moscow's annexation of Crimea on Thursday as Europe's leaders also readied to hit back at Russia with fresh sanctions.

President Vladimir Putin's spokesman condemned the new anti-Russian sanctions as unacceptable, and noted that some of the names on the blacklist caused "bewilderment."

Russian Foreign Minister Sergei Lavrov on Thursday told U.S. Secretary of State John Kerry that Moscow will not revisit its decision to incorporate Ukraine's peninsula of Crimea into Russia.

"S. Lavrov stressed that the decision to reunite Crimea and Russia which reflects an expression of will of an absolute majority of its residents is not subject to revision and should be respected," the foreign ministry said.

French President Francois Hollande said Europe's leaders would cancel a June summit with Russia and decide fresh sanctions against Russian figures as he arrived Thursday for a European Union summit.

Saying events in Ukraine and Crimea were "unacceptable", the French leader said "sanctions will be decided against a certain number of figures, regarding their personal situation or their financial assets".

A pro-Kremlin channel has aired a sensationalist report based on bugged phone calls and eavesdropped hotel meetings, accusing Russian protest leader Alexei Navalny of taking money from abroad and having contacts with the CIA.

The report was broadcast by the NTV channel, owned by state-controlled gas giant Gazprom, late Wednesday, one day after President Vladimir Putin had warned over a "fifth column" in Russia after its taking of Crimea.