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.

Russia's lower house of parliament on Thursday ratified the treaty incorporating Crimea into Russian territory, in defiance of the international community's insistence that the peninsula is part of Ukraine, with just one deputy voting against.

The vote in the State Duma lower house was the penultimate legislative hurdle for the treaty, which was signed on Tuesday by President Vladimir Putin and Crimea's leaders. The Duma also approved a new law on the absorption of Crimea into Russia.

German Chancellor Angela Merkel said Thursday that the EU would target more figures with travel bans and assets freezes over the Ukraine crisis, and also warned of economic sanctions.

Merkel told parliament that Russia's absorption of Ukraine's Crimea region, which has been condemned as a violation of international law, demanded a "resolute and united" response by Europe and its partners.

European leaders were on Thursday to debate biting economic sanctions against Russia for its annexation of Crimea as Ukraine tore up key ties with the Kremlin and drew up plans to evacuate its nationals from the rebel peninsula.

The European Union is under intense pressure to find a credible response to an explosive security crisis on the 28-nation bloc's eastern frontier that NATO chief Anders Fogh Rasmussen on Wednesday called "the gravest threat to European security and stability since the end of the Cold War."

Ukraine's security chief said on Wednesday he had instructed the foreign ministry to introduce visas for Russians in response to the Kremlin's claim on its flashpoint Crimea peninsula.

"The foreign ministry has been instructed to introduce a visa regime with the Russian Federation," Ukraine's National Security and Defense Council chief Andriy Parubiy told a televised press conference.

NATO's secretary-general said Wednesday that Russia's intervention in Ukraine posed the most serious threat to Europe's security since the end of the Cold War and warned Moscow it would face international isolation.

"This is a wake-up call," alliance chief Anders Fogh Rasmussen said in prepared remarks. "For the Euro-Atlantic community. For NATO. And for all those committed to a Europe whole, free and at peace."