Washington DC - The US will "speak out" to South Korea to safeguard democracy after President Yoon Suk Yeol's short-lived imposition of martial law, National Security Advisor Jake Sullivan said Wednesday.

"South Korea's democracy is robust and resilient, and we're going to continue to speak out publicly and engage privately with South Korean counterparts to reinforce the importance of that continuing," Sullivan said at the Center for Strategic and International Studies.

He said that Yoon's imposition of martial law "raised deep concern for us," and he praised the National Assembly for operating "according to constitutional processes and procedures" to rescind it.

"What we want to see is just the proper functioning of the democratic institutions of the ROK," he said, referring to the South by its official name, the Republic of Korea.

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Sullivan said that President Joe Biden's administration first learned of Yoon's declaration of martial law via television, repeating that the US was not consulted by its close ally.

His remarks follow a posting from the National Security Council that praised South Korea's "democratic resilience."

Yoon is now facing widespread demands to resign and potential charges of insurrection as thousands of protesters took to the streets on Wednesday.