sftp user:pass@host:22 -i /path/to/id_rsa

usage: sftp [-1246aCfpqrv] [-B buffer_size] [-b batchfile] [-c cipher]
          [-D sftp_server_path] [-F ssh_config] [-i identity_file] [-l limit]
          [-o ssh_option] [-P port] [-R num_requests] [-S program]
          [-s subsystem | sftp_server] host
       sftp [user@]host[:file ...]
       sftp [user@]host[:dir[/]]
       sftp -b batchfile [user@]host