A radio interview on Wednesday failed to bring clarity

He said that despite the aborted murder inquiry and missing persons search, the FSB - the successor to the Soviet-era KGB - knew where he was all the time.

Speculation was rife after Mr Rybkin went missing on 5 February, with press rumours that he was dead or kidnapped.

But he told Moscow radio on Wednesday he had gone to the Ukraine to escape the attention of the security services.

I truly felt danger to my personal security and even to my very life Ivan Rybkin

He remained evasive during the hour-long interview and the precise circumstances surrounding his disappearance are no clearer.

He said he had left his wife some money and food and had taken the train to Kiev because he was sick and tired of being followed by the security forces.

He said he had also switched off his mobile.

'Game'

But he said Ukrainian border guards had checked his passport and he had dealt with other officials during his absence, so he could easily have been found.

"All of this happened with the knowledge of the Russian secret service," he said.

"They announced all over the country that I was missing, that they can't find me. What kind of a game is this? I truly felt danger to my personal security and even to my very life."

As a head of Russia's Security Council under former President Boris Yeltsin, Mr Rybkin added that he knew how the agencies work.

Mr Rybkin, linked to exiled tycoon Boris Berezovsky, said he would take a week's break from the election campaign to decide whether to stay in the race for the presidency.

But he said he would not be taking part in any of the television debates with other candidates.

Putin critic

The BBC's Steve Rosenberg in Moscow said this was a surprising decision from someone who, until recently, was eager to voice public criticism of the Kremlin.

Mr Rybkin, a liberal candidate, had not been seen as a threat to Mr Putin in the 14 March election.

Upon arriving at Moscow's Sheremetyevo airport on Tuesday, he described the events of the last five days as the most difficult experience during his 15 years in politics.

He said it was like returning from a "difficult round of Chechen negotiations".

"I'm very satisfied that I returned," Mr Rybkin said, hinting at a possibility that he might not have come back alive.

Viktor Kurochkin, of Liberal Russia, said Mr Rybkin had complained about being followed before and his drivers had had to shake off suspicious cars.

A nationwide search for Mr Rybkin was started at the weekend, after his wife Albina and his campaign manager reported him missing to the Moscow police.

Russian prosecutors briefly opened a murder inquiry on Monday but cancelled it within an hour, citing a lack of evidence.