A gun turret pointed towards Kiev, a bunch of pro-Russia militiamen and a queue of impatient drivers: this strip of land near the village of Chongar is Russia's new de facto border.

More than 30 cars were backed up on the Crimean side, with the Black Sea on either side, and double that number can be seen on the Ukrainian mainland.

The White House on Friday billed President Barack Obama's Europe trip next week as a chance to show the United States leading a campaign to isolate Russia over its annexation of Crimea.

Obama will meet G7 leaders and confer with the leadership of European Union nations and NATO, as he seeks to impose costs on Moscow for its takeover of the Ukrainian region.

Tensions between Russia and the United States over Crimea could torpedo their nuclear cooperation, experts said Friday, despite the White House's insistence that both countries would move ahead on the critical issue.

"It could become a major problem indeed," Ken Luongo of the Partnership for Global Security said of the explosive Ukraine-Crimea crisis, the worst between Russia and the West since the Cold War.

Russia's annexation of the Crimea peninsula could cost Kiev hundreds of billions of dollars, Ukraine's interim prime minister Arseniy Yatsenyuk said on Friday, quoted by Interfax news agency.

Speaking to reporters in Kiev after signing a deal in Brussels on closer ties with the EU, Yatsenyuk said Russia was seizing "dozens of installations" from Ukraine worth "not billions, but hundreds of billions of dollars".

France has suspended most of its military cooperation with Russia, the French defense minister said Friday, as Paris was still mulling whether to nix the sale of two Mistral warships to Moscow.

Paris has suspended "the majority of its military cooperation with Russia" in the wake of Moscow's takeover of Crimea, Defense Minister Jean-Yves Le Drian said in the Estonian capital Tallinn.

U.N. chief Ban Ki-moon said on Friday the Ukrainian crisis can only be resolved through a diplomatic solution that respects the territorial integrity of the splintered ex-Soviet state.

Ban arrived in Ukraine a day after holding talks in Moscow with Russian President Vladimir Putin during which he expressed his deep concern over the worst East-West crisis since the Cold War.

President Vladimir Putin on Friday sealed the absorption of Crimea into Russia after the upper house of parliament unanimously ratified the treaty in the face of international condemnation.

In a televised ceremony in the Kremlin, Putin signed the legal documents then received a standing ovation from lawmakers and the singing of the Russian anthem.

President Vladimir Putin on Friday said Moscow would hold off on further reciprocal sanctions against the United States, after Washington introduced punitive measures against his close allies over the Ukraine crisis.

"You know, both in the first case -- the American sanctions -- and in the second case -- the introduction of a visa regime with Ukraine -- I think we should for now hold off on reciprocal steps," Putin told a meeting of his Security Council.

Iran's supreme leader Ayatollah Ali Khamenei said Friday that only a strong nation could avoid being oppressed by foreign powers, as he called for economic and cultural independence.

"A nation that is not strong will be oppressed," Khamenei, the country's top decision-maker, said Friday in the northeastern city of Mashhad in an address in honor of the Persian New Year.

Ukraine's interim premier Arseniy Yatsenyuk on Friday called for global economic pressure against Russia to halt its drive to forge "a new world order."

Speaking after the signature of a landmark EU-Ukraine partnership deal, Yatsenyuk said economic pressure was the only way to contain what he described as Moscow's unprecedented disregard for internationally agreed borders.