Yoon Suk Yeol
© Im Hun-jung/Yonhap via APSouth Korean President Yoon Suk Yeol
South Korean officials said Sunday they will "come up with our response accordingly" after revelations that the U.S. reportedly spied on its close ally and gathered signals intelligence related to South Korea's internal debate over weapons sales to the U.S., and Seoul's fears that those weapons would ultimately end up in Ukraine.

Officials in Seoul said they'll raise the alleged spying — which came to light as part of a major leak of sensitive documents over the weekend — with their U.S. counterparts.

For both countries, the timing is delicate. South Korean President Yoon Suk Yeol is scheduled to visit Washington and join President Biden for a state dinner at the White House on April 26.

"We will review precedents and instances involving other countries, and come up with our response accordingly," a South Korean presidential official said Sunday after being asked about the revelations, according to the country's Yonhap News Agency.

The New York Times first reported the leak Friday.

The document dump contained more than 100 images of government papers with classification markings posted to social media sites, including Twitter.

The documents, most of which are several months old, contained information about U.S. weapons provided to Ukraine, the disposition of Ukrainian forces, and sensitive intelligence regarding several other nations, including Israel and South Korea.

On South Korea, the documents appear to show internal debates about the sale of South Korean artillery shells to the U.S. It appears as if those conversations were intercepted by U.S. intelligence services and then summarized for review.

Seoul has reportedly insisted that the U.S. be the "end user" of military aid, as the country so far has resisted sending lethal assistance directly to Ukraine for its war with Russia. The leaked documents include apparent conversations between Mr. Yoon's secretary for foreign affairs, Yi Mun-hui, and South Korean National Security Adviser Kim Sung-han about the issue.

The government, Mr. Yi said, "was mired in concerns that the U.S. would not be the end user if South Korea were to comply with a U.S. request for ammunition," according to one document.

Those alleged comments suggest that Seoul feared the U.S. would send the South Korean artillery to Ukraine, circumventing South Korea's policy against providing military equipment directly to countries engaged in active conflict.

Those conversations were picked up through signals intelligence, The New York Times reported, which would essentially mean that the U.S. has been spying on its ally, perhaps by eavesdropping on phone conversations or through other means.

The documents include recaps of other internal discussions about how South Korea should handle the matter, including holding off on direct phone conversations between Mr. Yoon and Mr. Biden until Seoul had staked out a firm position.

"Yi stressed that South Korea was not prepared to have a call between the heads of state without having a clear position on the issue, adding that South Korea could not violate its policy against supplying lethal aid, so officially changing the policy would be the only option," one of the documents said, according to The New York Times.

That conversation appears to have taken place before U.S. and South Korean officials announced the state dinner.

The leak is likely to be a major embarrassment for the Biden administration and also seems to represent a significant national security breach. Some of the leaked documents were marked "Top Secret," the nation's highest classification marking.

Many of the documents have since been deleted, though open-source intelligence sleuths have been able to download more than 60 of the papers.

The Pentagon said late Friday that it is investigating the matter. The FBI also has reportedly launched an investigation into the leak.