Welcome to our coverage of fast-changing events in Ukraine, where the army is on full combat alert amid fresh diplomatic efforts to avert further escalation of the crisis and persuade Russian troops currently de facto controlling Ukraine's Crimea region to return to their bases.
There were some strong statements at an emergency meeting of the UN Security Council, which ended about an hour ago.
The meeting was called by Moscow. The Russian envoy to the UN, Vitaliy Churkin, produced a copy of the letter which he said had been signed by ousted Ukrainian President Viktor Yanukovych, asking Russia to deploy troops in Ukraine.
Ukraine's envoy Yuriy Sergeyev told the gathering that Kiev was yet to receive an official answer from Moscow about "why are the military forces of Russia are illegally occupying Crimea".
Other Security Council members - including Britain, China, France and the US - urged Russia to respect Ukraine's territorial integrity.
America's envoy Samantha Power asked the Russian representative: "Why choose military action when the consequences will be so devastating?"
In Crimea, Ukrainian troops remain blocked in their bases by Russian soldiers, who - according to Kiev - are now numbering 16,000 across the peninsular.
Ukraine says its forces in Crimea have been given until 03:00 GMT to surrender to Russia's military or face an assault - a claim denied by Moscow. Here, armed are seen standing outside Ukraine's base in Bakhchisaray.
emails: My reaction is fear and anger. Fear of the war, because we do not want any more deaths, because I have friends and relatives in Russia, they are not the enemies. But I'm angry, because the Russian government does not respect our rights and our choice. They are threatening us with their army. They already blocked airports in Crimea and do not let anyone go into Crimea. These are not peaceful actions!