Russia threatened late Wednesday that it could alter its position at Iranian nuclear talks in response to pressure from the European Union and United States over its seizure of Crimea.

"We would not like to use these talks as an element of a stakes-raising game taking into account the moods in various European capitals, in Brussels and Washington," deputy foreign minister Sergei Ryabkov told the Interfax news agency.

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."

Russia will build a rail and road bridge from Crimea to southern Russia, President Vladimir Putin said Wednesday, confirming the long-planned project would go ahead after Russia seized the peninsula.

"Here we need a bridge to take both cars and trains," Putin said at a meeting with his ministers, cited on the Kremlin website.

German Foreign Minister Frank-Walter Steinmeier said he had told his Russian counterpart Wednesday that approval is needed immediately for a mandate sending foreign observers to Ukraine, as Berlin halted a major arms deal with Moscow.

"I said again this morning in the conversation that the mandate really needs to go through in the next 24 hours," Steinmeier told reporters after telephone talks with Sergei Lavrov and the chairman of the Organization for Security and Cooperation in Europe (OSCE), Didier Burkhalter.

U.S. Vice President Joe Biden on Wednesday said Washington could send forces to the Baltic states to reassure ex-Soviet NATO allies rattled by Russia's takeover of Crimea.

"We are exploring a number of additional steps to increase the pace and scope of our military cooperation, including rotating U.S. forces in the Baltic region to conduct ground and naval exercises and training missions," Biden said during a visit to Lithuania, after a first stop in Poland.

British Prime Minister David Cameron said Wednesday that G7 nations meeting next week must discuss the permanent expulsion of Russia from the wider G8 if it takes further action to destabilize Ukraine.

U.S. President Barack Obama has called for a G7 summit -- minus current G8 chairman Russia -- on the sidelines of a nuclear security summit in The Hague on Monday and Tuesday to discuss the escalating showdown over Russia's annexation of Crimea.

Russia said Wednesday the United States had renounced its "co-sponsor" role in negotiating peace in Syria by closing down the Syrian embassy in Washington.

"By making such a unilateral move, our American partners in essence are depriving themselves of the role of co-sponsor of the process of political regulation in Syria and willy-nilly playing into the hands of the hard-core Syrian opposition," the foreign ministry said in a statement.

Pro-Russian forces captured Ukraine's naval commander after seizing his headquarters in Crimea on Wednesday as Moscow's grip tightened on the peninsula despite Western warnings its "annexation" would not go unpunished, as Kiev said it would start pulling its troops out of the disputed region.

Later on Wednesday, Russian forces seized control of a second Ukrainian navy base in western Crimea, Agence France Presse reporters saw, hours after capturing the main navy headquarters in Sevastopol.

Australia on Wednesday said it will impose targeted financial sanctions and travel bans against officials involved in the annexing of Crimea, rallying behind allies the United States and the European Union.

While none of the 12 Russian and Ukrainians in the firing line was named, Foreign Minister Julie Bishop said they were "individuals who have been instrumental in the Russian threat to the sovereignty and territorial integrity of Ukraine".