The US is calling for a smooth and peaceful transition in South Korea, saying the alliance between the two countries is crucial to regional stability and security.

"The United States continues to be a steadfast ally, friend and partner to the Republic of Korea. We certainly look forward to working with Prime Minister Hwang in his new capacity as acting president," said Mark Toner, Deputy Spokesman, US department of State.

"We expect and we believe that policy and continuity across a range of fronts, including DPRK is paramount, as well as international economics and trade."