North Korea appears to be preparing a missile launch to celebrate the anniversary of the birth of the country's founder Kim Il-Sung.
One or two Musudan ballistic missiles were deployed around the eastern port of Wonsan some three weeks ago, according to Yonhap news agency which quoted an unnamed Seoul official.
The move came as U.S. intelligence warned the North's ballistic missile capability is expanding.
The founder's birthday is an annual spectacular for North Korea, which celebrates with huge military parades featuring its most impressive-looking weapons or with missile launches.
"There is an ample possibility that the North would launch them around Kim Il-Sung's birthday," Yonhap quoted the South Korean official as saying.
The nuclear-armed state has staged several short- and mid-range missile launches but has yet to test the Musudan missile, known to have a range of up to 2,485 miles.
Seoul's defense ministry spokesman also said there was a "possibility" the North would carry out such a missile test around Friday's anniversary.
On Tuesday, senior U.S. politician Brian McKeon told a U.S. Senate hearing that North Korea's nuclear and missile program posed a growing threat to the United States and its allies in East Asia.
He said North Korea was trying to develop longer-range nuclear ballistic missiles capable of hitting the U.S. and was working to make its KN-08 road-mobile ICBM operational.
At the same hearing, Admiral Bill Gortney, the officer in charge of defending U.S. air space, said current assessment showed it was unlikely that North Korean missiles could hit the U.S., but it was prudent to assume it had the capability.
"We don't base our readiness levels on that low probability ... We are prepared to engage that particular threat," he said.
"Eventually, we assess that this low probability will increase, that's why the investment to have us outpace that technology is absolutely critical."
Gortney said he agreed with a South Korean assessment that North Korea was capable of putting a nuclear warhead on a medium-range missile that would reach all of South Korea and most of Japan.
Tension has been running high on the divided peninsula since the North conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.