The United States must shift its military focus from Iraq to Afghanistan, the country's top military official said Monday.

"The main effort in our strategic focus from a military perspective must now shift to Afghanistan," Joint Chiefs Chairman Adm. Mike Mullen said Monday after a trip to the Middle East and South-Central Asia. "This isn't about can-do anymore, this is about must-do, and we must do more over at least the next two years, starting now."

Mullen signaled an increased urgency for concentrating on Afghanistan and Pakistan amid a resurgence of Taliban and al-Qaeda forces in the region.

"I'm gravely concerned about the progress they have made in the south and inside Pakistan," he said. "The consequences of their success directly threaten our national interests in the region and our safety here at home."