Acting President Han Duck-soo speaks with US President Joe Biden by phone at the Prime Minister's office in Jongno-gu, Seoul. (The Office for Government Policy Coordination, Prime Minister's Secretariat)

In a 16-minute phone call, Acting President Han Duck-soo and US President Joe Biden on Sunday reaffirmed the "ironclad" alliance between Seoul and Washington, amid growing uncertainties surrounding the security situation on the peninsula, the Prime Minister's office said.

The talks came in the morning after the passage of an impeachment motion against President Yoon Suk Yeol in the National Assembly on charges of insurrection tied to his botched declaration of martial law on Dec. 3. Prime Minister Han stepped in as acting president immediately after Yoon was suspended from office late in the evening, in accordance with the Constitution.

"Our government plans to carry out diplomatic and security policies without disruptions, as well as make efforts to maintain and develop the ROK-US alliance without wavering," Han told Biden, according to the Prime Minister's office.

"It is crucial that we solidify the ROK-US combined defense posture more than ever, as both (countries) face North Korea's advancing missile threats and a deepening alliance between Moscow and Pyongyang," Han added.

Han reassured Biden that all state affairs will be managed based on the Constitution and the rule of law, emphasizing that no one, including the government, is above the law.

In response, Biden said that the "ironclad" alliance between South Korea and the US remains unchanged, said Han's office. The US president pledged to work closely with South Korea on the strengthening and continued development of the decades-old alliance between the two countries and the cooperation between Seoul, Washington and Tokyo.

Biden also expressed confidence and trust in South Korea's democracy after thanking Han for providing an explanation of the current situation.

South Korea currently faces both domestic and external risks related to Yoon's impeachment. Political turmoil in the country is projected to escalate as the Constitutional Court proceeds with its expected deliberation on whether to reinstate Yoon to power or formally oust him. The process could take up to 180 days and if Yoon is removed from office, a snap election will be called.

Several senior defense and military officials have stepped down or been arrested in recent weeks, accused of insurrection or colluding with Yoon in insurrection. This has led to growing concerns over the South's ability to maintain its military deterrence against North Korea's advancing nuclear and missile threats.