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.

OSCE member states agreed Friday to send to Ukraine a monitoring mission initially numbering 100 people after Russia dropped objections, but they will not have access to Crimea.

The Organization for Security and Co-operation in Europe will "deploy a special monitoring mission of international observers to Ukraine", a resolution passed by the OSCE's 57 member states said.

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".

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.

France will offer to send four fighter jets to the Baltic states and Poland to boost NATO air patrols over the region, a source close to the French defense minister said Friday.

Besides the Rafale and Mirage 2000 aircraft, Paris will also offer to ensure "AWACS patrols from France around twice a week" if requested, the source said during the minister's visit to Estonian capital Tallinn amid the Ukraine crisis.

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.

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.

Japan plans to provide nearly $1.0 billion in financial aid to Ukraine, public broadcaster NHK said Friday, as relations with Russia cool over the Crimea crisis.

Prime Minister Shinzo Abe is to announce the support when he attends a Group of Seven meeting in the Netherlands on the sidelines of a nuclear-security summit next week, NHK said.