Russian President Vladimir Putin said he was ready to put his country's nuclear forces on alert when he annexed Ukraine's Crimean peninsula last year in case of intervention by the United States and its allies.

"We were ready to do that," Mr Putin said when asked in a documentary film about Russia's takeover of Crimea on Russian state television if the Kremlin had been prepared to place its nuclear forces on alert. Mr Putin said he warned the US and Europe not to get involved, accusing them of engineering the ouster of Russian-backed Ukrainian President Viktor Yanukovych. "That's why I think no one wanted to start a world conflict."

In the film, Crimea: the Road to the Motherland, broadcast by Rossiya-1, Mr Putin said he sent military intelligence and elite navy marines to spearhead the disarmament of 20,000 Ukrainian troops in the territory. No date was given for the Putin interview. The film was made over eight months.

Vladimir Putin's seizure of Crimea in March last year provoked the worst geopolitical confrontation with the West since the Cold War. Photo: Reuters

Russia's seizure of Crimea in March last year provoked the worst geopolitical confrontation with the US and Europe since the Cold War. Tensions have escalated during a pro-Russian insurgency in eastern Ukraine that has killed more than 6000 people over the past year. Despite a European-brokered ceasefire, the US is considering arming Ukrainian forces.

Mr Putin, 62, whose country has been hit by US and European Union sanctions that have helped to drive the Russian economy toward recession, branded President Barack Obama's administration as "puppet-masters". He said the US directed the months of mass protests that overthrew Mr Yanukovych in February last year.