The United States has said it was "taking all the necessary precautions" after North Korea moved a medium-range missile to its east coast. The UN Secretary General meanwhile has called the North's threats "alarming."
North Korea's action Thursday was the latest in a series of escalating rhetoric. The country is angry over new UN sanctions and South Korea-US military drills, continuing to issue threats including those of nuclear war.
White House spokesman Jay Carney said threats coming from the North fitted a "regrettable but familiar" pattern of North Korean behavior.
"We're taking all the necessary precautions," Carney said, citing "prudent measures" in response to possible military threats.
Speaking from Madrid, UN Secretary General Ban Ki-moon expressed his concern over North Korea's latest actions.
"Nuclear threat is not a game," Ban said Thursday in Madrid. "It's very serious and I think they have gone too far in the rhetoric. I am concerned that if by any misjudgment, by any miscalculation of the situation, a crisis happens in the Korean Peninsula. This really would have very serious implications."
Missile cannot reach US
Earlier in the day, South Korean defense minister Kim Kwan-Jin said the missile the North moved to its east coast could reach a "considerable distance" but not the US mainland. As for the reason for the move, Kim said he could only speculate that it was for the purpose of "testing or drills."
On Wednesday, the Pentagon announced plans to step up security, particularly for its troops in the Pacific region by sending ground-based missile interceptors to be deployed on the US territory of Guam.
The Terminal High Altitude Area Defense system (THAAD) "will strengthen defense capabilities for American citizens in the US territory of Guam and US forces stationed there," the Pentagon statement said.
Kaesong plant shut again
Meanwhile on Thursday, North Korea for a second consecutive day prevented South Koreans from crossing the border to work at the Kaesong Industrial Park, where more than 100 mainly small South Korean firms operate, employing around 50,000 North Koreans.
Pyongyang has allowed South Korean managers to leave the industrial part, located five kilometers (three miles) inside North Korean territory. However, hundreds have chosen to stay, fearing they may not be able to return to run their businesses for the foreseeable future.
In recent days, North Korea has threatened to shut down Kaesong entirely over what it sees as "hostile" joint military exercises carried out by the US and South Korea.
dr,pfd/lw (AFP, Reuters, AP, dpa)