svnsync, windows with svn+ssh

I'm trying to run:

svnsync init file:///d:/SVN/ubiq-serv3/test
svn+ssh://svn_backup_serv3/svn/test

from a windows machine, to create a backup from a linux machine.. But I get:

"svnsync: Can't create tunnel: The system cannot find the file specified."

From within Putty.exe, I can run the session "svn_backup_serv3" ok, and I
get the svnserve process feedback from the server. I'm guessing this is
complaining about some internal issue between svnsync and putty?

Adrian Marsh