Ukraine has ordered a full military mobilisation in response to Russia’s build-up of its forces in Crimea.
Prime Minster Arseniy Yatsenyuk said the country was “on the brink of disaster”.
US President Barack Obama has called Russian troop deployments a “violation of Ukrainian sovereignty”.
Nato is conducting emergency talks on the crisis. Its secretary-general has said Russia’s actions “threatened peace and security in Europe”.
Several other measures were announced on Sunday by national security officials:
The armed forces would be put on “full combat readiness”.
Reserves to be mobilised and trained
Ukraine’s foreign minister will seek the help of US and UK leaders in guaranteeing its security
Emergency headquarters to be set up
Increased security at key sites, including nuclear plants.
Airspace closed to all non-civilian aircraft.
The BBC has seen what appear to be Russian troops digging trenches on the Crimean border.
Heavily armed groups continue to occupy key sites on the peninsula, including airports and communications hubs, although there has been no actual violence.
Tensions are high in Crimea, with several stand-offs reported around Ukrainian military bases.
One Ukrainian base was surrounded by Russian troops in Perevalnoe, south of the regional capital Simferopol, with another base in Sevastopol being blockaded by a pro-Russian “self-defence unit”.
In the Crimean 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.
Observers have been watching a build-up of Russian military activity in Crimea – home to Russia’s Black Sea Fleet – for the past few days.
Reports say two Russian anti-submarine warships have appeared off the coast of Crimea in violation of an agreement governing the presence of Russia’s Fleet in the peninsula.
Russian soldiers are widely reported to be guarding a number of administrative buildings and military bases in Crimea. Parliament, airports, the state television building and telecommunications hubs have also been surrounded.
Some 6,000 extra Russian troops and 30 additional armoured vehicles are now in Crimea, Ukrainian Defence Minister Ihor Tenyukh said on Saturday.
The newly-elected pro-Moscow leader of Crimea, Sergiy Aksyonov, said he had 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.
On Saturday, Mr Obama held a 90-minute telephone conversation with Russian leader Vladimir Putin and urged him to pull forces back to bases in Crimea.
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.
The US, France and Canada say they are suspending preparations for a G8 summit in the Russian resort of Sochi in June as a reaction to the deployment, with Canada also recalling its ambassador from Moscow for consultations.
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”.