bzr push sftp with publickey

[ITSec_Mike]

Hi all

I would like to use bzr push sftp://

The server I want to connect to needs publickey authentication.
How can I provide the identity file to the above command?

Obviously the following does not work:
bzr push sftp://server/dir
Permission denied (publickey,keyboard-interactive).
bzr: ERROR: Unable to connect to SSH host <ip_address>; EOF during negotiation

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any help is very much appreciated.
Thanks,
Mike