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PARIS, Aug 16 (Reuters) - Russia must withdraw from all major towns in Georgia under a peace accord it has signed, despite conditions authorising "additional security measures," according to a letter sent by French President Nicolas Sarkozy to Georgian leader Mikheil Saakashvili.

"As I specified at our joint press conference in Tbilisi, these 'additional security measures' can only be implemented in the immediate proximity of South Ossetia to the exclusion of any other part of Georgian territory," read the letter, made public by Sarkozy's office on Sunday.

The French-brokered agreement drafted this week authorises Russian forces to take extra security measures on a temporary basis pending the arrival of international peacekeepers -- which requires a U.N. Security Council resolution.

Uncertainty remains over how quickly and how far Russian forces will be withdrawn and Georgia accused Russian troops on Saturday of severing the country's main east-west train link by blowing up a railway bridge in broad daylight.

Russia has denied that its forces were responsible for the bridge's destruction.

Sarkozy's letter said that under the accord, Russian forces would not be authorised to remain in any major towns outside South Ossetia and road and rail transport should be guaranteed.

"More precisely, these 'measures' may only be implemented within a zone of a few kilometres from the administrative limits between South Ossetia and the rest of Georgia, in such a way that no major urban centre is included -- I am thinking in particular of the town of Gori," the letter read.

"Special arrangements will have to be defined to guarantee freedom of movement along the road and rail routes of Georgia," it said. (Writing by James Mackenzie; Editing by Jon Boyle)

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