South Korean authorities believe North Korea could carry out an intercontinental ballistic missile system test as soon as this week, which would seriously raise tensions on the Korean Peninsula as the presidential transition gets underway in Seoul.

Seoul and Washington have detected signs of an imminent ICBM test, and have been keeping an eye on the developments, according to sources here.

National Security adviser Suh Hoon told President-elect Yoon Suk-yeol on Saturday that a test launch was imminent, and it would not be a surprise if the North fired it on Monday, according to a Chosun Ilbo report on Monday, citing an unnamed official at the president-elect’s office.

An official at Seoul’s Joint Chiefs of Staff said Monday that is difficult to make a call on when the test will take place, but stressed that the authorities are keeping close tabs on the situation and maintaining a robust readiness posture.

Weather conditions and other factors are likely to decide the timing of Pyongyang’s possible launch.

In a rare joint announcement, Seoul and Washington said last week that the North’s two recent missile launches on Feb. 27 and March 5 -- which Pyongyang claimed were for a reconnaissance satellite -- were to test a new ICBM system.

The new missile system, known as the Hwasong-17, was first unveiled during a military parade in October 2020, with some analysts calling it a “monster” for its size.

While the recent launches only tested parts of the missile disguised as a satellite, the Pentagon said that the North may conduct an ICBM test at “full range” in the future.

Such a test would be in defiance of Pyongyang’s self-imposed four-year moratorium on testing nuclear weapons and long-range ballistic missiles. Amid the stalled nuclear talks with the US, the North in January threatened to no longer observe the moratorium. The North last tested an ICBM, the Hwasong-15, in 2017, which was assessed as capable of reaching the US mainland.

Meanwhile, US special envoy to North Korea Sung Kim has called on Beijing to join Washington in publicly condemning the North’s recent missile launches, according to the State Department on Sunday.

The call was made during a phone conversation between Kim and his Chinese counterpart Liu Xiaoming on Thursday, the day when Washington revealed its intelligence that the North recently tested a new ICBM system.

During their conversation, Kim told Liu he was concerned that the recent launches demonstrate the North’s “determination to advance its unlawful weapons of mass destruction and ballistic missile programs and continue an increasingly escalatory path.”

Kim also encouraged China to urge the North to “cease its destabilizing activity and return to dialogue.”

The North’s recent escalation of tension comes as South Korea elected Yoon Suk-yeol of the conservative People Power Party as its new president. Yoon is expected to take a hawkish stance on Pyongyang, and has raised the possibility of taking a preemptive strike on the North if a nuclear attack was imminent.

The North also appears to be repairing underground tunnels at the Punggye-ri nuclear test site, which it dismantled several years ago, in a sign that the regime could resume nuclear tests again.

The Unification Ministry, which is in charge of inter-Korean affairs, on Monday urged the North to immediately cease actions that “run counter to peace and stability on the Korean Peninsula and are of no help to the development of inter-Korean relations” and return to dialogue, according to its spokeswoman Lee Jong-joo.

Meanwhile, top nuclear envoys of Seoul, Washington and Tokyo also spoke via phone Monday, condemning the North’s series of ballistic missile launches as a violation of the United Nation Security Council resolutions, the Foreign Ministry said in a statement.

The three countries urged the North to stop activities that raise tension on the peninsula and return to dialogue as soon as possible.