WASHINGTON (Kyodo) -- The United States is watching unfolding developments in South Korea with "grave concern," Deputy Secretary of State Kurt Campbell said Tuesday, following South Korean President Yoon Suk Yeol's sudden but short-lived declaration of martial law.

Although the White House released a statement saying it is "relieved" the decree was lifted, there is uncertainty over the political stability of one of Washington's most important security allies, with calls growing for Yoon to step down.

The fluid situation in South Korea comes before President Joe Biden, who has been proud of his efforts to deepen ties with democratic-elected leaders like Yoon, leaves office in January and with North Korea showing no sign of giving up its nuclear ambitions.

"We have every hope and expectation that any political disputes will be resolved peacefully and in accordance with the rule of law," Campbell said.

The second-ranking U.S. diplomat, while attending an event at the State Department, said officials are seeking to engage with their South Korean counterparts at every level.

A National Security Council spokesperson said in a statement, "We are relieved President Yoon has reversed course on his concerning declaration of martial law" adding that "democracy is at the foundation" of the U.S.-South Korea alliance.

Defense Department spokesman Pat Ryder said the United States was not notified by South Korea in advance of the martial law declaration.

Ryder told a press briefing that the decree, which was formally lifted about six hours after taking effect, has not impacted U.S. troops based in South Korea.

He also said, "We're closely monitoring the situation, but I'm not aware of any force posture changes," when asked whether the Pentagon had seen any indications that North Korea was poised to take advantage of the situation.

Yoon's surprise decision, announced in a late-night address to the nation, came at a time when his and Biden's administrations have been working with Japan to expand three-way cooperation in the face of shared security challenges, including China's rise.

The move stunned the United States and other nations as it was South Korea's first martial law declaration in more than 40 years, with the last dating back years before the country's democratization in 1987.

Yoon, struggling with low approval ratings, said martial law was needed to eradicate pro-North Korea "anti-state" forces. He accused the country's main opposition Democratic Party, which controls parliament, of trying to overthrow the liberal democratic system.

His announcement of the emergency step, apparently taken without circumstances that warranted it, triggered large demonstrations in Seoul and prompted South Korea's parliament to hold a swift vote to lift the declaration.

Facing criticism from almost all quarters, including from his own political party, Yoon acquiesced, accepting the vote and withdrawing troops that had been deployed to the parliamentary building.

Under the martial law order, all parliamentary and other political activities are prohibited, and the freedom of the press is restricted.

Amid ongoing political chaos in South Korea, Campbell, an architect of the Biden administration's Asia approach, said he wanted to underscore that the bilateral alliance remains "ironclad" and that the United States stands by the country "in their time of uncertainty."

In mid-November, the leaders of Japan, the United States and South Korea met on the sidelines of an international summit in Peru and announced the creation of a secretariat to further align their policies and actions.

The development was part of a last-minute bid to "institutionalize" the trilateral framework before U.S. President-elect Donald Trump returns to power in January with what is likely to be his go-it-alone approach to foreign affairs.