fetching 'rsync' output with nodejs child_process.exec callback

Currently I'm failing to fetch the rsync output when I'm calling nodejs child_process.exec with a callback-function like in this snippet:

var sys = require('sys'),
exec = require('child_process').exec;

cmd = 'rsync -rpz test/test-files/one.txt jloos@test.mygnia.de:~/remote-test/a/b/'
exec(cmd, function(error, stdio, stderr) { 
  sys.print('s: ' + stdio + '\n');
  sys.print('e: ' + stderr + '\n'); 
});

I think this is caused by the specific behavior of rsync. rsync communicates with it's counterpart via terminal. So how can I fetch the messages from rsync, if even possible?

When I use cmd = 'ls -la' I get the expected output.

Thanks