SEOUL (Reuters) – North Korea has deployed one or two intermediate-range ballistic missiles on its east coast, possibly preparing for launch on or around Friday, the anniversary of the birth of the country’s founder, South Korea’s Yonhap news agency reported.

A mobile launcher was spotted carrying up to two Musudan missiles, Yonhap said on Thursday, citing multiple South Korean government sources, following North Korea’s fourth nuclear test in January and a long-range rocket launch the next month, which led to fresh U.N. sanctions.

The Musudan missile, with a design range of more than 3,000 km (1,800 miles), is not known to have been flight-tested, according to South Korean defense ministry and experts.

Some experts said North Korea may choose to test-fire the Musudan in the near future as it tries to build an intercontinental ballistic missile designed to put the mainland United States within range.

U.S. intelligence believes North Korea’s ability to reach the United States is low, but its capabilities will increase, making continued investment in missile defense essential..

South Korea’s Defence Ministry spokesman, Moon Sang-gyun, declined to confirm the Yonhap report but said the military had been on high alert for any North Korean missile launch since its leader Kim Jong Un’s vow to conduct more tests.

Kim said in March his country would soon test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads.

The United States, which has 28,000 troops stationed in South Korea, said it was aware of reports that North Korea was preparing to test intermediate-range missiles and was closely monitoring the Korean Peninsula

“We call again on North Korea to refrain from actions that further raise tensions in the region,” said Pooja Jhunjhunwala, a State Department spokeswoman.

Admiral Bill Gortney, the officer responsible for defending American airspace, told a U.S. Senate hearing on Wednesday that 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.

The United States and South Korea began talks on possible deployment of a new missile-defense system, the Terminal High Altitude Area Defense (THAAD), after the latest North Korea nuclear and rocket tests.

North Korea, which regularly threatens to destroy South Korea and the United States, often fires missiles during periods of tension in the region or when it comes under pressure to curb its defiance and abandon its weapons programs.

South Korean experts have said North Korea may choose to display a show of force ahead of a major ruling party congress in May where it is expected to declare itself a nuclear power or around the April 15 anniversary of the birth of Kim’s grandfather, Kim Il Sung.

(Reporting by Ju-min Park in Seoul and David Brunnstrom in Washington; Editing by Jack Kim, Nick Macfie and Jonathan Oatis)