Philip Crowley explained that should the south vote to secede, it is only six months from that point to where a new nation will emerge.
He added that there no time to waste because many complex and important issues have to be resolved.
Border demarcation and share of energy resources are among the remaining major points.
U.S. envoy to the Sudan Scott Gration is in Sudan to meet government leaders in Khartoum and Juba to accelerate the implementation of the remaining articles of the CPA, including the referendum.