But will they fight? They have not so far.
2 March 2014 Last updated at 09:43 ET
Ukraine orders full military mobilisation over Russia moves
Ukrainian soldiers were surrounded by Russian troops at Perevalnoe, one of several stand-offs on the peninsula
Ukraine has ordered a full military mobilisation in response to Russia's build-up of its forces in Crimea.
Prime Minister Arseniy Yatsenyuk said Ukraine was "on the brink of disaster".
In Crimea, Ukrainian soldiers faced off with Russian soldiers surrounding their bases while the Russian army is said to be digging trenches on the border with mainland Ukraine.
US Secretary of State John Kerry has warned Russia could be ejected from the Group of Eight developed nations.
You just don't in the 21st Century behave in 19th Century fashion by invading another country on completely trumped up pretext”
End Quote John Kerry US secretary of state
Russian President Vladimir Putin, he said, was "not going to have a Sochi G8, he may not even remain in the G8 if this continues," referring to a planned summit in Russia in June.
"He may find himself with asset freezes, on Russian business. American business may pull back, there may be a further tumble of the rouble.
"You just don't in the 21st Century behave in 19th Century fashion by invading another country on completely trumped-up pretext," Mr Kerry told the CBS program Face the Nation.
The facts on the ground are such that Russia, to a large extent, is already in control there ”
Jonathan Marcus BBC diplomatic correspondent
The UK has joined the US, France and Canada in suspending preparations for the Sochi summit.
Nato is conducting emergency talks, saying Russia's actions threaten "peace and security in Europe".
US President Barack Obama called Russian troop deployments a "violation of Ukrainian sovereignty".
Russian soldiers continue to occupy key sites on the Crimean peninsula, including airports and communications hubs, although there has been no actual violence.
In Moscow, police detained dozens of people at anti-war rallies outside the Russian defence ministry and at other spots in the city centre.
Ukrainian national security officials announced several other measures on Sunday:
At the scene
Oleg Boldyrev BBC Russian, Feodosia
Crowds of excited pro-Russian Crimeans have gathered near the gates of the marines' base in Feodosia. A deadline demanding the marines pledge loyalty to the new government in Crimea has passed. Despite threats to attack the base, local Cossacks, who act as law and order here, continue to stand in a chain near the gates. Behind the gates I could see freshly-made dugouts, but no-one is inside them at the moment. A couple of marines in sand-coloured flak jackets are manning the gates from the inside.
The street leading to the base is blocked by two armoured personnel carriers, with another closer to the gates. When asked whose APCs and soldiers they were, a Cossack said "Russians", but he didn't know whether they came from Black Sea Fleet base in Sevastopol or Russia. From time to time the loudspeaker on a van near to the entrance of the base calls on the marines to recognise the new pro-Russian government in Crimea, and not to act upon "criminal orders from Kiev".
Any filming activity in the crowds is risky - we saw cameramen and photographers being harassed. One cameraman had to leave the approach to the base and another was led away by a policeman.
- The armed forces would to be put on "full combat readiness"
- Reserves to be mobilised and trained
- Foreign minister to seek help from US and UK leaders in guaranteeing its security
- Emergency headquarters to be set up
- Security to be boosted at key sites, including nuclear plants
- Airspace to be closed to all non-civilian aircraft
Meanwhile, Ukraine withdrew coast guard vessels from two ports in Crimea and moved them to other bases in the Black Sea and Sea of Azov.
One Ukrainian army base was surrounded by Russian troops in Perevalnoe, south of the Crimean regional capital Simferopol, with another base in Sevastopol being blockaded by a pro-Russian "self-defence unit".
In the eastern port city of Feodosia, a group of Ukrainian marines were also blockaded into their base.
Armed men surrounded the base demanding that the garrison pledge loyalty to the region's new pro-Russian authorities.
Around 100 marines are thought to be inside the base.
In the north of Crimea, at Armyansk on the Isthmus of Perekop, the BBC saw what appeared to be Russian soldiers digging trenches. The isthmus is strategically vital as it joins Crimea to the rest of Ukraine.
Russian ground troops have been active in Crimea - home to Russia's Black Sea Fleet - for the past few days.
On Saturday the newly elected pro-Moscow leader of Crimea, Sergiy Aksyonov, appealed to Mr Putin for help to ensure peace on the peninsula.
The interim government in Kiev does not recognise Mr Aksyonov and his government, and signed a decree on Saturday that their election at an emergency session of the regional parliament this week was illegal.
Russian soldiers are apparently digging trenches on the strip of land joining Crimea to mainland Ukraine
This Russian convoy was moving from Sevastopol to Simferopol on Sunday
Many ethnic Russians in Crimea have held pro-Russian demonstrations in recent days
Kharkiv is one of several cities that has seen clashes between pro- and anti-Russian protesters in recent days
Meanwhile pro-EU protesters rallied once again in the capital Kiev
"Ukrainians - I am ashamed to be a Russian!" - placard of protester arrested in Moscow
Late on Saturday, Mr Obama held a 90-minute telephone conversation with Mr Putin and urged him to pull forces back to bases in Crimea.
President Obama had a lengthy telephone call with President Putin, as Simon Clemison reports
Mr Putin said Moscow reserved the right to protect its interests and those of Russian speakers in Ukraine.
President Putin submitted his request for troops to the Russian parliament on Saturday "in connection with the extraordinary situation in Ukraine and the threat to the lives of Russian citizens", the Kremlin said.
Ukrainian Prime Minister Arseniy Yatsenyuk says his country is "on the brink of disaster"
Mr Obama, the White House said, told Mr Putin that the appropriate way to address any concerns "is peacefully through direct engagement" with the Ukrainian government and international mediating bodies.
He told Mr Putin his actions were a "breach of international law, including Russia's obligations under the UN Charter, and of its 1997 military basing agreement with Ukraine", a statement added.
The Kremlin said that in his phone call with Mr Obama, President Putin "underlined that there are real threats to the life and health of Russian citizens and compatriots on Ukrainian territory".
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