SEOUL (Reuters) -South Korean investigators again asked the country's acting president on Saturday to order the presidential security service to comply with an arrest warrant for impeached President Yoon Suk Yeol.

The plane crash is the first major test for South Korea’s acting president, Choi Sang-mok, who was appointed the interim leader only on Friday, as the country grapples with a political crisis at the highest levels.

South Korea's acting President Choi Sang-mok vowed to continue trilateral cooperation with Japan and the United States, saying on Friday his country's diplomacy remains steadfast despite an unprecedented political crisis at home.

South Korea’s main opposition party said Tuesday it will seek to impeach acting leader Han Duck-soo after he missed an opposition-set deadline to approve independent investigations into impeached President Yoon Suk Yeol and his wife.

South Korea's opposition-controlled National Assembly voted Friday to impeach acting President Han Duck-soo despite vehement protests by governing party lawmakers, further deepening the country's political crisis set off by President Yoon Suk Yeol's stunning imposition of martial law and ensuing impeachment.

The impeachment of Han Duck-soo, acting president since Yoon was impeached on Dec. 14 for declaring martial law, has thrown South Korea's once-vibrant democratic success story into uncharted territory.

South Korean lawmakers voted on Friday to impeach acting president Han Duck Soo, barely two weeks after Yoon Suk Yeol was removed from the presidency over his imposition of martial law. A total of 192 lawmakers in the opposition-controlled parliament voted to impeach Han - significantly more than the simple majority required.

South Korean Acting President Choi Sang-mok called for national harmony and unity and for trust in the government in a New Year's address on Tuesday, amid an unprecedented political crisis sparked by impeached President Yoon Suk Yeol's short-lived martial law declaration.