(Adds Hague's comments, paragraphs 13-14, Juncker remarks, paragraph 28)
* Ukraine forced to cede naval base, flagship, submarine
* EU foreign policy chief urges help for Ukraine's economy
* Britain says allies should mull arms sale cuts to Russia
By Aleksandar Vasovic and Gabriela Baczynska
BELBEK AIRBASE, Crimea, March 22 (Reuters) - Russian troops used armoured vehicles, automatic gunfire and stun grenades on Saturday to seize a Ukrainian airbase in Crimea a day after President Vladimir Putin signed laws completing Russia's annexation of the Black Sea peninsula.
Ukrainian forces also abandoned a naval base after attacks by pro-Russian protesters, and had to surrender two flagship vessels to Russian forces.
The facilities at Belbek and Novofedorovka had been among the last still under Ukrainian control after Moscow's armed takeover and subsequent annexation of Crimea, which has a majority ethnic Russian population and harbours one of Russia's biggest naval bases at Sevastopol.
Russia's seizure of the peninsula after the ousting of Ukraine's pro-Russian president by mass protests has triggered the worst East-West crisis since the Cold War. The United States and the European Union have targeted some of Putin's closest long-time political and business allies with personal sanctions.
At the Belbek airbase, a Reuters reporter said armoured vehicles had smashed through the walls of a compound and that he had heard bursts of gunfire and grenades, making the takeover one of the more dramatic of Russia's largely bloodless occupation of Crimea.
Russian forces had already seized Belbek's airstrip and warplanes at the start of the crisis. The compound seized on Saturday contained barracks, arms depots and a command building.
Colonel Yuliy Mamchur, the base commander, said a Ukrainian serviceman had been injured and that he himself he was being taken away by the Russians for talks at an unspecified location. He said the Ukrainians were placing their weapons in storage in the base.
NAVAL ASSETS SURRENDERED
Ukraine's naval base at Novofedorovka, near Sevastopol, was vacated after unarmed pro-Russian protesters attempted to force their way in, Ukrainian military spokesman Vladislav Seleznyov said in a Facebook post.
Russian troops also seized the flagship of the Ukrainian fleet, the Slavutich, which had been prevented from leaving port by Russian tug boats, he said, adding that Ukraine's only submarine, the Zaporizhya, had been taken by Russian forces on Friday.
There have been few casualties since Russian forces started seizing control of military facilities in Crimea, though one Ukrainian serviceman was killed and two others were wounded in a shooting in Simferopol earlier this week.
Ukraine's Defence Ministry said on Friday that Crimea's bases were still formally under its control, but most are now occupied by Russian troops and fly Russia's tricolour flag.
Firework displays on Friday in Crimea and Moscow marked Russia's formal annexation of the peninsula. But Ukrainian residents in Crimea were mostly staying behind closed doors, with some packing their bags to leave.
British Foreign Secretary William Hague said in an article posted on the website of the Telegraph newspaper on Saturday that Britain and its allies should consider imposing lasting limitations on arms sales as part of a new relationship with Russia following the "outrageous" annexation of Crimea.
"This would involve Russia being outside some international organisations, facing lasting restrictions on military cooperation and arms sales, and having less influence over the rest of Europe," Hague wrote.
During a visit on Saturday to Kiev and the eastern city of Donetsk, German Foreign Minister Frank-Walter Steinmeier praised the interim prime minister, Arseny Yatseniuk, for statements aimed at reassuring the Russian-speaking majority in eastern Ukraine.
Later, after meeting business leaders in the east, Steinmeier said he believed they backed Ukrainian unity and would oppose any attempts to split the region off.
In addition to its concerns over Crimea, Ukraine's new government has said it desperately needs cash to cover expenses including gas imports and avert a possible debt default.
Yatseniuk said Ukraine would need energy from the European Union to protect it from the repercussions of its standoff with Moscow, on which it depends for over half its oil and gas.
On Friday, Yatseniuk signed a landmark association agreement with the EU, committing Ukraine to closer political and economic cooperation with the 28-nation bloc.
European Union foreign policy chief Catherine Ashton urged support for Ukraine's economy, above all:
"We have to make sure that Ukraine, economically, does not fall over," she said in a speech. "My biggest fear right now is the state of economy and the need for us all to offer the support that they need."
The Obama administration has asked Congress to approve a $1 billion loan guarantee package for Ukraine. The EU has said it is willing to provide $15 billion in loans and grants over several years. The EU aid is contingent on widespread reforms in Ukraine, and the signing of a deal between Kiev and the International Monetary Fund.
U.S. President Barack Obama's national security adviser said on Friday that the world was reassessing its relationship with Russia, and that Washington was sceptical of Russian assurances that troop movements on Ukraine's border were merely exercises.
Disagreement flared on Saturday over a six-month monitoring mission to Ukraine by the Organisation for Security and Cooperation in Europe.
The mission will be deployed in nine places outside Kiev, but there is no specific mention of Crimea in its mandate.
Russia's Foreign Ministry said on Saturday that the mandate "reflects the new political and legal realities and does not apply to the Crimea and Sevastopol, which became part of Russia".
But a German government spokesman said the OSCE was "expressly not casting into doubt the territorial integrity of Ukraine".
Jean-Claude Juncker, the former Luxembourg premier who wants to be next head of the European Commission, said Europe must rush through an association deal with Moldova to avoid it becoming Putin's next target for annexation. (Additional reporting by Alessandra Prenticel, Adrian Croft, Sabine Siebold, Alastair Macdonald, Stephen Brown and Costas Pitas; Writing by Will Dunham and Frances Kerry; Editing by David Gregorio)
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