Heavily-armed unidentified troops in Simferopol Heavily armed men - not identified by any insignia - have been occupying key sites in Crimea. Photo: BBC NEWS

US President Barack Obama has called Russian troop deployments a "violation of Ukrainian sovereignty".

Ukraine has said it will seek the help of US and UK leaders in guaranteeing its security. Nato has called emergency talks to be held at 1200 GMT.

Several other measures were announced by Andriy Parubiy, chair of the national security and defence council of Ukraine:

• The armed forces would be put on "full combat readiness".
• Reserves to be mobilised and trained
• Emergency headquarters to be set up
• Increased security at key sites, including nuclear plants.

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.

Russian 'violation'

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".