There are limits to how effective additional troops dispatched to Afghanistan can be, Joint Chiefs Chairman Mike Mullen said Wednesday morning, indicating the military won't request more troops than its requested.

Mullen signaled that dispatching any more than the 21,000 additional troops heading to Afghanistan will be adequate, and more troops will not be necessary to stabilize that country and its border with Pakistan.

"I think we know enough, in terms of what it's going to take," Mullen told NBC's "Today" show this morning. "There are limits on how many troops we can provide to make a difference."

"Well, there's no question in my mind that these 21,000 troops are absolutely vital," Mullen added, arguing that the central Asian front hadn't had enough troops to maintain stability.

Mullen, the top military official in the U.S. said that the next two years of fighting in Afghanistan and Pakistan would show whether or not there's potential for success in the extended military conflict.