Admiral Samuel Locklear, the commander of U.S. forces in the Pacific region, said the U.S. military believed North Korea had moved an unspecified number of Musudan missiles to its east coast.
An Obama administration official, speaking on condition of anonymity, told Reuters "our working assumption is that there are two missiles that they may be prepared to launch". That was in line with South Korean media reports.
The North has been threatening the United States and its "puppet" South Korea on an almost a daily basis in recent weeks, although the threats appear to be aimed partly at boosting internal support for young leader Kim Jong-un.
The Combined Forces Command in Seoul raised its "Watchcon 3" status, a normal defense condition, by one level in order to step up monitoring and increase the number of intelligence staff, a senior military official told the Yonhap news agency in the South on Wednesday.
"There are clear signs that the North could simultaneously fire off Musudan, Scud and Nodong missiles," Yonhap quoted an unidentified official as saying.
South Korean Foreign Minister Yun Byung-se told a parliamentary hearing in Seoul that he was working through diplomatic channels in an attempt to rein in Pyongyang.
"Through close coordination with China and Russia, the Korean government has been continuing to make efforts to persuade North Korea to change its attitude," Yun said.
China is North Korea's sole major ally, although it backed recent United Nations resolutions against Pyongyang, and Moscow was a supporter of North Korea as the Soviet Union.
UNKNOWN QUANTITY
Pyongyang has frequently tested short-range Scud missiles but the longer-range Musudan and Nodong missiles are an unknown quantity. The Musudan missiles are reckoned to have a range of roughly 3,000-3,500 km (1,865-2,175 miles).
The North has said it would target American bases in the Pacific, although it is not known whether the untested missiles have the range to do so.
"If the missile was in defense of the homeland, I would certainly recommend that action (of intercepting it). And if it was defense of our allies, I would recommend that action," Locklear told a Senate hearing in Washington.
Pyongyang has turned up its shrill rhetoric in recent weeks after the United Nations Security Council imposed sanctions for the impoverished state's third nuclear weapons test in February.
It has threatened a nuclear strike on the United States - something it does not have the capacity to carry out - and "war" with South Korea.
On Tuesday, it told foreigners in South Korea to leave the country to avoid being dragged into a "thermonuclear war". It previously warned diplomats in Pyongyang to prepare to leave.
The streets of Seoul, a city of 10 million people, bustled as normal on Wednesday morning as commuters traveled to work in sunny, spring-time weather. Foreign embassies in the capital of Asia's fourth-largest economy have played down the latest North Korean threats as rhetoric.
The North closed a money-spinning joint industrial park it operates with South Korean companies this week, putting at risk a venture that is one of its few sources of hard cash.
Analysts say the current tensions will likely last until the end of April, when joint U.S.-South Korean military drills end. The harsh rhetoric also precedes the first anniversary of Kim's formal ascent to power in Pyongyang.
The North has termed the drills "hostile" preparation for invasion by Seoul and Washington, who say the drills are regular annual exercises.
