South Korea to Probe Circumstances Around Reported Leak of Classified U.S. Documents

SEOUL—A South Korean official said the government was investigating the circumstances around a leak of highly classified U.S. documents that appeared to show intelligence based on intercepted communications of U.S. allies, including discussions among Seoul officials about concerns over selling ammunition to the U.S. that could end up in Ukraine.

South Korea’s first priority was to establish the facts, an official from the country’s presidential office said on Monday, adding that the reports of the intelligence leak hadn’t been confirmed. Once that is done, South Korea “plans to request, if necessary, the U.S. to take reasonable measures,” the official said.

The official said the process would be “based on the relationship of trust established between the two countries.” Other U.S. allies have been affected by the leak, including Israel, France and the U.K., and South Korea would “look into how other countries are responding” to the matter, the official said.

The U.S. is seeking to assess the damage from the intelligence breach after dozens of images of purportedly classified U.S. documents began circulating online. A State Department spokesman declined Monday to comment on the validity of the documents. He said the U.S. was engaging at high levels with allies and partners over the recent intelligence leak to reassure them of Washington’s commitment to safeguarding intelligence and to the security of their partnerships.

The Wall Street Journal hasn’t been able to independently verify the authenticity of the documents, but they contain enough detail to give them credibility.

On Tuesday, the official from the presidential office said the defense ministers from the U.S. and South Korea agreed that a significant number of the documents were forged, without offering further details. The official also dismissed the possibility that another country could eavesdrop on officials in the South Korean presidential office, saying the security was ironclad.

The documents, viewed by the Journal, include purported details of a conversation that took place on March 1 between top aides to South Korean President Yoon Suk Yeol, in which they discuss a sale of ammunition to the U.S. and concerns about it violating the country’s policy against providing lethal aid if the U.S. wasn’t the end user.

South Korea, one of the world’s fastest-growing arms manufacturers, has come under pressure from Western officials to supply weapons directly to Ukraine. The country signed its largest-ever arms export deal last year, agreeing to supply tanks and jet fighters to Poland. Last month, South Korea said it had approved the exports of components that were part of Krab howitzers made in Poland, though noting that its policy against providing lethal aid to Kyiv hadn’t changed.

The two former South Korean officials cited in the documents—Yi Mun-hui, then-secretary for foreign affairs at the National Security Council, and Kim Sung-han, then-national security adviser—discussed concerns in the National Security Council that President Biden would call Mr. Yoon directly before Seoul had a “clear position” on supplying the ammunition, the documents said.

Messrs. Yi and Kim stepped down from their posts at the end of March, and South Korea hasn’t publicly offered an explanation for their departures. Asked whether the government had confirmed the discussion between the two former officials or investigated them over the matter, the official from the presidential office said the government doesn’t customarily confirm details of diplomatic and intelligence issues, without elaborating further. Messrs. Yi and Kim couldn’t be reached for comment on Monday.

According to the documents, Mr. Kim expressed concern about how it might be perceived domestically if South Korea were to announce a change in its position on supplying lethal aid around the same time as the two countries announced a state visit by Mr. Yoon to Washington. Mr. Kim said there was a risk the public would think the countries had struck a deal, the documents said.

Mr. Yoon is scheduled to visit Washington for a state visit on April 26. The visit was announced by the White House on March 7.

Mr. Kim suggested the possibility of selling 330,000 rounds of 155mm ammunition to Poland “as getting the ammunition to Ukraine quickly was the ultimate goal of the U.S.,” the documents said. Mr. Yi agreed that it might be possible for Poland to agree to being called the end user and send the ammunition on to Ukraine, but that South Korea would need to verify what Poland would do, the documents said.

South Korea’s first deputy national security adviser, Kim Tae-hyo, is scheduled to visit the U.S. this week. He is expected to discuss the recent U.S. document leak while there, among other matters such as preparations for the South Korean president’s state visit and North Korea.

Write to Jiyoung Sohn at jiyoung.sohn@wsj.com