Camera Moving Dependless On World Axis

Hi,

I'am trying to move the camera likewise the Microsoft's 3dViewer app right click. My purpose is moving the camera just on xy axis, however when I rotate the camera xy is zooming the target object, because the world's coordinate is also changing.

I guess that this is an issue about local vs world coords. However I couldn't find that how I should solve this problem.

Here is my sample code :

      if (Input.GetMouseButton(1))
      {
          x += Input.GetAxis("Mouse X") * xSpeed * 0.02f;
          y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;
      }
      x = Mathf.Clamp(x, xMinDistance, xMaxDistance);
      y = Mathf.Clamp(y, yMinDistance, yMaxDistance);
      vectorRightClick = new Vector3(x, y, 0);
      Vector3 position = rotation * zoomDistance + target.position + vectorRightClick;
      transform.position = position;

Thanks for looking.