Washington DC - The White House on Tuesday said the US was "relieved" after South Korea's President Yoon Suk Yeol said he would lift martial law just hours after imposing it.

"We are relieved President Yoon has reversed course on his concerning declaration of martial law and respected the ROK National Assembly's vote to end it," a National Security Council spokesperson said in a statement, using the acronym for South Korea's official name.

The White House earlier said it was "seriously concerned" by the developments in South Korea – a key US ally – and was not notified in advance of Yoon's announcement, in which he accused the opposition of threatening the country's democracy.

President Joe Biden was briefed on the situation in South Korea during his visit to Angola, where he arrived on Monday for his first trip to sub-Saharan Africa as president.

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The US embassy in Seoul said it was cancelling routine appointments on Wednesday as a precaution. Despite Yoon lifting martial law, "the situation remains fluid," an embassy notice said.

Washington stations about 28,500 troops in South Korea to protect it from North Korea, its nuclear-armed neighbor.

The US and South Korea have long carried out joint exercises, infuriating the North, which views them as rehearsals for invasion and has frequently conducted weapons tests in retaliation.

Biden has also fostered the relationship between Washington, Seoul and Tokyo as a bulwark against both North Korea and an increasingly assertive China.

Earlier Tuesday, the US State Department echoed the White House concern, with Deputy Secretary of State Kurt Campbell stating that Washington has "every hope and expectation that any political disputes will be resolved peacefully and in accordance with the rule of law."

"I do want to underscore that our alliance with the ROK is ironclad, and we stand by Korea in their time of uncertainty," he said.

Yoon has been a close ally of the US, with Biden welcoming him on a state visit last year and South Korea in March hosting a global democracy summit, a signature idea of the outgoing US administration.

In the wake of Donald Trump's election victory in November, the South Korean president congratulated the Republican, saying he hoped to work closely with the incoming US administration.