North Korea is threatening to attack U.S. fighter jets in retaliation to sanctions from the U.S. and U.N.
Those sanctions came after North Korea's nuclear test in February.
North Korea's official news agency says it's restarting a nuclear reactor that could create more fuel for nuclear weapons. The country has said it's at war, but the U.S. sees little evidence of that.
North Korea is conducting target practice on pictures of U.S. soldiers, showing new animation of them shooting down U.S. planes.
"It has ratcheted up the danger," said Secretary of Defense Chuck Hagel.
The White House says so far it's all talk, no action.
"Despite the harsh rhetoric we're hearing from Pyongyang, we are not seeing changes to the North Korean military posture," said White House Press Secretary Jay Carney.
But the U.S. isn't taking in chances. As of this morning we've got Stealth Fighter jets and bombers overhead, and a Navy destroyer headed that way.
"You have to let em know that we're around and that even if they consider these rhetorical flourishes," said Former U.S. Ambassador to U.N. Bill Richardson.
"I don't think we can take anything for granted. The United States, South Korea and our other partners in that region have to be very prepared in the event that something could happen here," said Former Secretary of Defense Leon Panetta.
South Korean officials held a security meeting overnight. People on the streets there don't seem worried.
"I really don't think people are thinking they will come actually attack us because, you know, America has got our back," said one South Korean citizen.
Kim Jong Un has cut off emergency ties to world leaders and says his missiles are ready, but he hasn't recalled workers from factories near the South Korean border.
Just the threat of war raises concerns. "People are concerned that a provocation gets out of hand, then there's a response, and then it escalates," said Senator Bob Corker, (R) Tennessee.
That's the concern this morning, along with the possibility that North Korea could sell its nuclear weapons to Iran.