South Korea's Acting President and Prime Minister Hwang Kyo-ahn plans to send a congratulatory message to incoming US President Donald Trump in time for his inauguration later this month, an informed source said Monday.

The plan came as Seoul is seeking to strengthen its alliance with Washington amid a string of diplomatic and security challenges such as Beijing's apparent retaliation about the planned deployment of a US anti-missile system to the peninsula and a reignited row with Tokyo over Japan's wartime sexual slavery issue, not to mention North Korea's escalating military threats.

"In line with customary practices, we plan to send a congratulatory message to President-elect Donald Trump," the source with knowledge of the matter told Yonhap News Agency over the phone, declining to be named.

His message to Trump is to be delivered around Jan. 20 when the new US administration takes office.

In his message, Hwang is expected to strengthen joint efforts to counter Pyongyang's evolving nuclear and missile threats.

In the nationally televised address on New Year's day, North Korean leader Kim Jong-un made a veiled threat that Pyongyang is close to mastering the technology for an intercontinental ballistic missile capable of reaching the US mainland. Pyongyang also said it can test-fire an ICBM "at any time and any place."

Seoul has sought to get off to a good start with the new Washington government amid concerns about policy shifts under Trump's presidency.