President Yoon Suk Yeol (left) and US Republican presidential nominee and former US President Donald Trump (Yonhap, AP-Yonhap)

President Yoon Suk Yeol sent a congratulatory message to Donald Trump on Wednesday evening Seoul time, when he was reported as on the brink of victory.

Yoon posted to his official account on social media platform X, "Under your strong leadership, the future of the ROK-US alliance and America will shine brighter. Look forward to working closely with you." ROK stands for Republic of Korea, which is South Korea's official name.

"Depending on the election outcome, Yoon is expected to have a chance to communicate with the US President-elect in the coming days," an official of the presidential office said on condition of anonymity, a few hours before former US president and Republican candidate Donald Trump delivered his acceptance speech.

With Trump declaring victory Wednesday Seoul time, South Korea is facing pressure to devise a new approach to the South Korea-US alliance -- which has practically served as the central axis of South Korea's foreign affairs strategy under the incumbent US President Joe Biden's administration -- the continuity of which is in question due to transition of power.

But the unnamed official said that they have maintained communication with the campaign teams of both Trump and his Democrat contender Kamala Harris, who is currently Vice President of the US.

During Biden's tenure in Washington, Yoon's office has lauded the two-way alliance between South Korea and the US, as well as the three-way ties between South Korea, US and Japan, as keys to countering security threats on the Korean Peninsula with North Korea's growing military provocation.

Although Trump often touted his relationship with North Korean leader Kim Jong-un as the only former US president to have held bilateral talks with a North Korean counterpart, the official of the presidential office said Trump's stance toward North Korea will likely change after a transition of power.

The official added that North Korea's deployment of troops to Russia to potentially engage in direct combat in the Ukraine war has made the situation very complicated.

Seoul is also closely watching the impact Trump's declaration of victory could have on the defense cost-sharing deal formally signed Monday, over the US armed forces' deployment to South Korea.

When asked about the potential renegotiation of the cost-sharing deal in a scenario where Trump returns to the White House, the official said communication has been taking place with both the Trump and Harris campaign teams over various agendas regarding the Seoul-Washington alliance. This includes the degree of South Korea's contribution to the cost to maintain 28,500-strong US Forces Korea.

Yoon is poised to hold a press conference and deliver an address to the nation on Thursday.