* Putin says Russia more concerned by Syria's fate than Assad's
* Russia calls for U.N. reaction to kidnapping of foreigners in Syria
By Alexei Anishchuk and Gabriela Baczynska
MOSCOW, Dec 20 (Reuters) - Any solution to the conflict in Syria must ensure President Bashar al-Assad's forces and his opponents do not simply swap roles and fight on forever, Russian President Vladimir Putin said on Thursday.
In what appeared to be his first direct comments on the possibility of a post-Assad Syria, Putin said Syrians would ultimately decide their own fate and called for a negotiated transition period instead of attempts for military victory.
"We are not concerned about the fate of Assad's regime. We understand what is going on there and that the family has held power for 40 years," Putin told a news conference.
"We are worried about a different thing - what next? We simply don't want the current opposition, having become the authorities, to start fighting the people who are the current authorities ... and (we don't want) this to go on forever."
The West and some Arab states accuse Russia of shielding Assad after Moscow blocked three U.N. Security Council resolutions intended to increase pressure on Damascus to end the violence that has already killed more than 40,000 people.
Moscow has been reluctant to endorse the "Arab Spring" popular revolts of the last two years, saying they have increased instability in the Middle East and created a risk of radical Islamists seizing power.
"We are for a solution being found to the problem that would save the region and the country from, firstly, falling apart, and from a never-ending civil war. Our position is not to keep Assad and his regime in power at any cost," Putin said.
Western powers and some Syrian opposition groups have said Russia is shifting its stance on Syria, a suggestion dismissed by Moscow.
A Moscow-based foreign policy analyst, commenting on Putin speech, said Russia was unlikely to change stance on Syria even though it realised Assad's government would eventually fall.
"We don't know how long Assad's regime will hold out - a month, six months, a year - but its end is unavoidable now," said Georgy Mirsky, a Middle East expert at the Institute for World Economy and International Relations.
"For Putin it's still easier to lose Syria (than change tack) because he will then say he was fighting for a peaceful, compromise solution to the very end."
Moscow insists it will not allow a repeat of last year's events in Libya, where NATO helped rebels topple Muammar Gaddafi after Russia abstained, rather than use its veto, in a vote on a U.N. Security Council resolution that was used as a pretext for military intervention.
On Thursday Russia's Foreign Ministry also called on the Security Council, long deadlocked on the Syrian conflict that erupted in March 2011, to urge the Syrian opposition to cease threats and attacks on foreigners in the country.
It has also called for the release of foreigners held in Syria, including two Russians kidnapped earlier this week.