Ukraine on Monday ordered its outnumbered troops to withdraw from Crimea after Russia's lightning annexation of the peninsula as world leaders met for emergency talks on the worst East-West standoff since the Cold War.

The dramatic announcement came after Ukraine lost yet another base in Crimea and as world leaders gathered in The Hague for a security summit set to be dominated by concerns over the Ukraine crisis.

Russia's deputy defense minister Anatoly Antonov denied on Sunday it was massing troops on the Ukrainian border, saying it was observing international agreements.

"The Russian defense ministry is observing all the international agreements on limiting the numbers of troops in regions bordering Ukraine," Antonov told journalists, cited by the Interfax news agency.

President Barack Obama opens a six-day trip to Europe and Saudi Arabia on Monday seeking to reassure U.S. allies amid the worst East-West crisis in years and to defend his nuclear diplomacy with Iran.

A visit to the third Nuclear Security Summit in The Hague -- which grew out of Obama's initiative to prevent the spread of weapons of mass destruction and radioactive material -- has been planned for months.

Canada's premier said Saturday that Russian President Vladimir Putin's decision to send troops into Crimea and annex it from Ukraine encourages aggrieved nations to "arm themselves to the teeth".

Stephen Harper, the first leader from the Group of Seven top industrialized powers to visit Kiev since last month's fall of Ukraine's pro-Kremlin regime, said the consequences of Putin's actions "will be felt far beyond the borders of Ukraine or even the European continent itself".

Russia on Saturday expressed hopes for the success of the OSCE monitoring mission in Ukraine, while stressing the observers were excluded from Crimea after its takeover by Moscow.

"The Russian side hopes that objective and unbiased work by international observers will help overcome the crisis inside Ukraine," the Russian foreign ministry said in a statement.

Russian president Vladimir Putin has "lost Ukraine for good" with his move to annex Crimea, former prime minister Yulia Tymoshenko said Friday in her first live television appearance since returning to the country.

Tymoshenko, who spent three years in jail on what she said were politically motivated charges before being freed last month, appealed to fellow Ukrainians to be ready to fight in the event of a Russian invasion.

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.

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."

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.

Former U.S. secretary of state Hillary Clinton on Tuesday accused Russian President Vladimir Putin of attempting to "rewrite the boundaries" of post-World War II Europe.

It came as Putin signed a treaty claiming the Black Sea region of Crimea as Russian territory, as Ukraine warned the showdown had entered a "military stage" after soldiers were killed on both sides.