The United States has expressed "grave concern" over the situation in South Korea following President Yoon Suk Yeol's martial law declaration on Tuesday, urging that political disputes be resolved peacefully and under the rule of law.

"We are watching the recent developments in the Republic of Korea (ROK) with grave concern," said US Deputy Secretary of State Kurt Campbell, emphasizing Washington's position on the crisis. 

He added, "We have every hope and expectation that any political disputes will be resolved peacefully and per the rule of law."

Despite escalating tensions, Campbell reaffirmed the United States' steadfast commitment to its alliance with South Korea. "Our alliance with the ROK is ironclad, and we stand by Korea in their time of uncertainty," he said. 

However, the US also voiced frustration over being caught off guard by the martial law declaration.

A National Security Council statement noted that the US had not been notified in advance, expressing serious concern about the situation on the ground.

"We are seriously concerned by the developments we are seeing," the statement said.

The US also emphasized the importance of the South Korean National Assembly’s recent vote to oppose the martial law declaration, urging that the democratic process be respected.

"We hope and expect that the laws and regulations of a particular country are abided by, including the National Assembly vote," said State Department spokesperson Vedant Patel.

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

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

The United Kingdom also expressed concern, with a Downing Street spokesperson stating that they are "closely monitoring developments in South Korea."

British nationals were advised to stay updated through the UK’s travel advice and follow local authorities' guidance.

China, the key ally of North Korea, with whom Seoul remains technically at war and whose leader Kim Jong Un has engaged in escalatory rhetoric and military posturing this year, also voiced concern about the declaration.

China’s embassy in Seoul issued a warning to its citizens, urging them to "exercise caution" amid the political unrest. The embassy advised Chinese nationals to remain calm, stay informed, and avoid unnecessary outings.

The Kremlin, too, expressed concern over the martial law declaration. Russian President Vladimir Putin’s spokesperson Dmitry Peskov said, "The situation is concerning. We are watching closely," signalling that Moscow is closely following the developments in South Korea.

The United Nations also voiced alarm, with Stephane Dujarric, spokesperson for UN Secretary-General António Guterres, stating, "We are monitoring the situation very closely and with concern."

Dujarric noted that "the situation is evolving quickly," as international stakeholders remain on high alert.

The crisis began when President Yoon Suk Yeol declared martial law on Tuesday, citing threats from the North and accusing the opposition of being "anti-state forces."

Lawmakers quickly voted to oppose the declaration, but the military sided with the president, creating a standoff with no clear resolution in sight.

On Wednesday, the South Korean National Assembly voted overwhelmingly to block the martial law declaration.

"Of the 190 present, 190 in favour, I declare that the resolution calling for the lifting of the emergency martial law has been passed," National Assembly Speaker Woo Won-shik announced. 

Despite the parliament’s decisive vote, South Korea's military stated that martial law would remain in effect until lifted by President Yoon," South Korean military officials told local media, including YTN.

This move has further fueled the ongoing political crisis in the country, with no immediate end in sight.