Kerry was kicking off four days of talks in East Asia amid speculation that the North's unpredictable regime would launch a mid-range missile designed to reach as far as the U.S. territory of Guam. Kerry also planned to visit China and Japan.
North Korea delivered a fresh round of rhetoric Thursday, saying it has "powerful striking means" on standby for a missile launch.
U.S. officials said there is new intelligence that North Korea could launch a missile at any moment. American troops in South Korea have been told the fuel has been loaded and the missiles are on the pad.
The Pentagon said a high-tech floating radar is now up and running and a flotilla of naval destroyers is already off the Korean coast for early detection.
Defense Secretary Chuck Hagel said Wednesday that North Korea's leader, Kim Jong Un, is so unpredictable that the U.S. has no choice but to prepare for every contingency. He made clear the U.S. is ready to respond.
"North Korea has been, with its bellicose rhetoric, with its actions...skating very close to a dangerous line," Hagel told reporters. "Their actions and their words have not helped defuse a combustible situation."
Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, offered a grim assessment of how close North Korea is to putting a nuclear warhead on a missile, which is a direct threat to the U.S.
"In the absence of concrete evidence to the contrary, we have to assume the worst case," said Dempsey.
In Seoul, anxiety is growing amid South Korean news reports that emergency threat levels jumped to Watchcon 2, which is one step away from war. Even as Koreans went about their daily routines, many wonder just how far North Korea's young leader will go.
If the launch happens, the missile would be a mid-range called the "Musudan," which has never before been tested in flight. It is powerful enough to travel more than 2,000 miles, putting U.S. bases in Okinawa or Guam within its reach. Its path could be over Japan, which is now on high alert and moving patriot missiles into position around Tokyo.
U.S. defense leaders say intelligence suggests that North Korea could be planning multiple launches. Many experts believe it will happen April 15, the nation's biggest holiday to celebrate the birthday of the late President Kim Il Sung, founder of North Korea and the grandfather of Kim Jong Un.
Meanwhile, on the streets of Pyongyang, the focus was on celebrating milestone anniversaries that highlight the Kim family's hold on power in North Korea.
After marking late leader Kim Jong Il's appointment to a top government post Tuesday, North Koreans were putting on their finest clothing to celebration his son Kim Jong Un's ascension to first secretary of the Workers' Party a year ago Thursday.
The Associated Press contributed to this report.