* PM issues some of Moscow's strongest criticism of Assad
* Says Assad's slow pace of reform may be "fatal mistake"
* But repeats that Assad mustn't be ousted by external forces (Adds quotes, details, context)
By Alexei Anishchuk and Steve Gutterman
MOSCOW, Jan 27 (Reuters) - Russian Prime Minister Dmitry Medvedev said Syrian President Bashar al-Assad's chances of retaining power are getting "smaller and smaller" every day, according to the transcript of an interview with CNN released by Medvedev's office on Sunday.
His remarks were the most vocal Russian statement that Assad's days could be numbered. But he reiterated calls for talks between the government and its foes and repeated Moscow's position that Assad must not be pushed out by external forces.
"I think that with every day, every week and every month the chances of his preservation are getting smaller and smaller," Medvedev was quoted as saying. "But I repeat, again, this must be decided by the Syrian people. Not Russia, not the United States, not any other country.
"The task for the United States, the Europeans and regional powers ... is to sit the parties down for negotiations, and not just demand that Assad go and then be executed like (the late former Libyan leader Muammar) Gaddafi or be carried to court sessions on a stretcher like (Egypt's) Hosni Mubarak."
Russia has been Assad's most important ally throughout the 22-month-old Syrian conflict, which began with peaceful street protests and evolved into an armed uprising against his rule.
Moscow has blocked three U.N. Security Council resolutions aimed at pushing him out or pressuring him to end the bloodshed, which has killed more than 60,000 people. But Russia has also distanced itself from Assad by saying it is not trying to prop him up and will not offer him asylum.
Medvedev made some of Russia's harshest criticism of Assad to date, placing equal blame for the escalation into a civil war on "the leadership of the country and the irreconcilable opposition". He also said Assad was far too slow to implement promised political reforms.
"He should have done everything much faster, attracting part of the moderate opposition, which was ready to sit at the table with him, to his side," Medvedev was quoted as saying. "This was his significant mistake, and possibly a fatal one."
The wording of the interview suggested it was not just Assad's grip on power that was under threat, but his life. Medvedev's remark about the chances of his "preservation" diminishing came when he was asked whether Assad could survive.
Russia has repeatedly called on Western and Arab nations to put more pressure on Assad's foes to seek a negotiated solution, but Medvedev acknowledged that Moscow's influence on the Syrian president is limited.
"I have personally called Assad several times and said: conduct reforms, hold negotiations," said Medvedev, who was Russia's president until last May. "In my view, unfortunately, the Syrian leadership is not ready for this.
"But on the other hand, by no means should a situation be allowed in which the current political elite is swept away by armed actions, because then the civil war will last for decades," he said.
Russia has given frequent indications it is preparing for Assad's possible exit, while continuing to insist he must not be forced out by foreign powers.
Russia sells arms to Syria and uses a naval facility on the Mediterranean coast that is its only military base outside the former Soviet Union.
But analysts say its policy is driven mainly by President Vladimir Putin's desire to prevent the United States from using military force or support from the U.N. Security Council to bring down governments it opposes. (Editing by Mark Heinrich)