Ukrainian border guards say Moscow has poured almost 20-thousand troops into crisis-hit Crimea.
That is on top of the 11-thousand already permanently based with Russian Black Sea fleet in the port of Sevastopol.
President Vladimir Putin denies that forces with no national insignia are under Moscow’s command. He has also rebuffed a US warning over military intervention.
Following a telephone conversation with Barack Obama, Putin reiterated that his country cannot ignore calls for help from Russian speakers in the region.
Obama announced the first sanctions against Moscow since the start of the crisis.
Meanwhile, unarmed military observers deployed by the Organisation for Security and Cooperation in Europe have been denied access to Crimea – after arriving in the Ukrainian port of Odessa.
The crisis escalated on Thursday, when Crimea’s regional government set a referendum on the peninsula’s status for 16 March.
The vote is coming under criticism from the West, but it has won Russian backing. One Russian parliament leader has stressed that Crimea has a right to self-determination.
Ukraine crisis – how it unfolded
24: Thousands protest in Kyiv over government move to shelve EU association agreement.
17: Ukraine secures a 11bn euro bailout from Russia.
19: Up to 200,000 gather in Kyiv to show opposition to newly-enacted anti-protest laws. Clashes between police and protesters.
20: Clashes continue into second day.
22: Kyiv Post reports five killed and 300 injured as clashes intensify.
23: Truce announced which paves the way for arrested protestors to be released.
28: Prime Minister Mykola Azarov resigns. Nine of 11 anti-protests laws repealed after vote in parliament. Stand-off continues between police and protesters in Kyiv.
18: 20,000 protestors march to parliament with MPs set to debate a possible new constitution. At least 17 people, including seven policeman, are killed as fresh clashes erupt.
19: Truce agreed.
20: Truce breaks down, fresh clashes see 48-hour death toll rise to at least 77.
21: Peace deal signed, with talk of early elections. Violence spreads to western Ukraine.
22: Protesters freely take control of presidential buildings amid reports Yanukovych has fled. Parliament votes to remove him with fresh elections set for May. Yanukovych appears on TV and denounces a “coup dêtat”. Opposition leader Tymoshenko released from jail.
23: Tymoshenko ally becomes acting president, saying European integration is a priority
25: Parliament votes for ousted Yanukovych to be tried at International Criminal Court.
26: Interim government moves to disband Ukraine’s riot police force as leaks lift the lid on the high-living of ousted president Yanukovych
27: Reports emerge suggesting Yanukovych is now in Russia as parliament appoints new pro-EU government. It comes amid fears of separatism after pro-Russian gunmen takeover government building in Crimea.
28: Yanukovych, speaking at a press conference, vows to fight for Ukraine, calls new government illegitimate and denies ordering police to fire on protesters. It comes as gunmen seize airports in Crimea.
1: Putin and Russian parliament back use of armed forces in Crimea – prompting US, UK, and Germany to condemn.
2: Ukraine calls up all its reservists and says country is on brink of disaster. NATO warns Russia over its military activities as Ukrainian navy commander defects to Crimea.
3: UK calls crisis the biggest of this century as Russia reportedly tells Ukrainain forces in Crimea to surrender. Rouble tumbles amid crisis, as foreign minister Sergei Lavrov defends Russia’s actions..
4: Putin says Russia has right to intervene but the use of force will be a ‘last resort’. Then, Russia, under diplomatic pressure from, among others, the US, agrees to talks.
5: The US and Russia hold crucial talks in Paris as tensions mount in Crimea. Ukraine’s new PM accuses Russia of a coup in Crimea as EC puts together much-needed funds for the crisis-hit country.
6: A UN envoy is forced out of Crimea before the region’s MPs vote to become part of Russia. US says any referendum in Crimea about joining Russia would be against international law.
7: Tensions continue in eastern Ukraine as US pushes for sanctions over Russia’s intervention in Crimea. Obama and Putin, after a crisis call, remain divided.