North Korea is also telling all foreign companies and tourists in South Korea to evacuate, raising fears the communist country will carry out its threats of nuclear war.
North Korea told foreign diplomats in Pyongyang last week that their safety could not be guaranteed starting Wednesday. It is not clear what significance that date holds. The country is preparing in Pyongyang to commemorate the100th birthday of the late President Kim Il Sung, founder of North Korea and the grandfather of the new leader, on April 15.
"Any future nuclear test or missile launch would be in direct violation of UN Security Council resolutions and would lead to further pressure and isolation," said Patrick Ventrell, the U.S. State Department's press office director.
However, analysts believe a direct attack on Seoul is highly unlikely, and there are no overt signs that North Korea's army is prepared for war, let alone a nuclear one. In Pyongyang, there were no signs of a military buildup. South Korea's military has reported missile movements on North Korea's east coast, but nothing pointed toward South Korea.
Such threats are, however, seen as rhetoric and an attempt by North Korea to scare foreigners into pressing their governments to pressure Washington and Seoul to change their policies toward Pyongyang, and to boost the military credentials of its young leader, Kim Jong Un.
"In a society that reveres the elderly, a 30-year-old untested novice leader, this is something he needs to demonstrate. He needs to demonstrate his manhood," said David J. Karl, an international relations professor at the University of Southern California.
Adm. Samuel Locklear, commander of U.S. Pacific Command, testified Wednesday on contingency plans.
Locklear told the Senate Armed Services Committee that he was confident that the U.S. military can thwart North Korea if it chooses to act.
"Do we have the capability to intercept a missile if the North Koreans launch within the next several days?" Sen. John McCain, R-Ariz., asked the admiral.
"We do," Locklear answered.
North Korean state-run TV is telling all South Koreans to flee because an attack is imminent.
"This kind of rhetoric will further isolate North Korea from the international community," said White House spokesman Jay Carney.
Many Koreans in the U.S. are paying close attention to the rising tensions.
"Here in Southern California, the largest Korean community in the United States, we all have family members back in Korea, so it's a nervous time for us," said John Cho, the international editor of the Korean Times, based in Koreatown. "We hope he's bluffing. But with Kim Jong Un, you can never be sure."
USC professor David Kang said both sides have clear incentives to avoid all-out war.
"The reason deterrence is stable is that North Korea knows that if they attack first, the regime ceases to exist. We know that if we attack first, Seoul as a city will cease to exist," said Kang.
A big question if North Korea fires a test missile is what direction they will fire it off. In the past, they've fired missiles out towards the ocean, away from people. But the possibility of them intentionally or unintentionally firing towards populated areas is terrifying for some in the region.
The U.S. has moved two of the Navy's missile-defense ships closer to the Korean peninsula, and a land-based system is being deployed to the Pacific territory of Guam.
Japan has prepared several missile interceptors at three sites around Tokyo as a precaution. Japan has taken similar measures before but has never actually tried to shoot down a North Korean missile.
The Associated Press contributed to this report.