MOSCOW (AFP) - Russian President Vladimir Putin said he was ready to put the country's nuclear forces on alert as he sought to annex Crimea last year after the fall of a Moscow-backed government in Kiev.

Speaking in a documentary aired on state TV in Russia on Sunday a year after the takeover of the Black Sea Peninsula from Ukraine, Putin showed the lengths he was willing to go to in order to protect Russian-speaking citizens of the province he considered a historic slice of Russia.

The documentary, Homeward Bound, features several interviews with Putin who explains how he gave "direct instructions" to the armed forces for a swift and bloodless mission to take back the peninsula that had been transferred from Russian to Ukrainian jurisdiction in 1954 by Soviet leader Nikita Khrushchev.

Putin also showed his level of confidence in the operation which took a matter of days, saying he had "no doubts" it would succeed.

Asked whether Russia was ready to put its nuclear forces on alert, Putin said: "We were ready to do this."

"I was speaking with colleagues and said frankly that this is our historical territory and Russian people live there, they were in danger, and we cannot abandon them," he said in comments recorded over several months.

"It wasn't clear right away," if the West would intervene militarily, said Putin.

His spokesman Dmitry Peskov said the leader, whose unusual 10-day absence from the public eye has created frenzied speculation in Russia, would watch the final version of the documentary for the first time on Sunday.

Putin explained how after Ukraine's Russian-backed president Viktor Yanukovych fled Kiev in the face of a pro-Western revolution, he organised a secret poll to test the sentiment of the Crimean population.

"But it was not us who carried out the coup d'etat. This was done by nationalists and people with extreme beliefs" in Kiev.

With support for absorption into Russia high, he sent men with unmarked uniforms across the peninsula, including Russian special forces, marines and airborne troops to neutralise the Ukrainian military.

"In order to block and disarm 20,000 well-armed men, you need a specific set of personnel. We needed specialists who know how to do it," said Putin.

He also explains how Russian troops worked for hours to convince troops in Crimea to cross over to their side or resign.

"We turned Crimea into a sea and land fortress." The annexation of Crimea was a critical event in the Ukrainian crisis which many believed triggered the separatist uprising in eastern Ukraine where over 6,000 have since been killed in fighting.

It also plunged Moscow's ties with the West to lows unseen since the Cold War.

The move struck a deep chord with Russians, however, cementing Putin's 15-year grip on power as his approval ratings have continued to climb in the past year, hitting a record 88 percent, according to figures released last week.

Putin said it had not occurred to him to try and take back Crimea until what he repeatedly describes as a "coup" by pro-Western protesters.

He describes how he initially advised Yanukovych not to leave Kiev, and if he did, not to call off his security forces who had clashed repeatedly with the protesters.

"'Oh, yes, yes, I know that,'" Putin said Yanukovych replied "only then to leave and pull all security forces out!"

Putin said he received countless reports that the pro-Western movement was plotting to kill Yanukovych and that the president's motorcade had at one point come under fire. He said Yanukovych tried to remain on Ukrainian territory but after a few days asked Putin to extract him.

"The fact that we saved his life, the life of his family members, I think it's a good deed, a noble one," said Putin.

Yanukovych - who was accused of ordering his forces to open fire leading to the deaths of some 100 people in the days before he fled the capital - allegedly told Putin he could not bring himself to give the order to use weapons.

"Whether it was good or bad, the consequences of inaction have been grave. That much is obvious," the Russian leader said.

Putin insisted his actions in Crimea prevented the kind of violence now seen in eastern Ukraine and said it was his "duty" and that he would do the same thing if presented with the same situation today.

"If you have the inner confidence that you are doing the right thing and that your actions aim to benefit the country and to defend the interests of the people of Russia, if this inner confidence is there, than everything will work out," Putin said.