South Korea's opposition urged the government on Wednesday to investigate alleged espionage by the United States after leaked documents appeared to show Washington spying on its key Asian ally.

A trove of highly sensitive US intelligence that has emerged online included revelations that Washington had been spying on President Yoon Suk Yeol’s national security advisors as part of an effort to secure arms supplies for Ukraine.

Seoul sought to downplay the importance of the leaked documents on Tuesday, with Yoon's office claiming "a significant number" of the documents were fake and his national security advisor saying there were no "malicious intentions" in the incident.

But the revelation has sparked criticism in South Korea about the vulnerability of sensitive sites including the presidential office.

"The government must get to the bottom of eavesdropping allegations and if they are found to be true, it must get an official apology and guarantee that it won't do it again from the US," Lee Jae-myung, head of the opposition Democratic party, said on Wednesday.

Opposition lawmakers have accused the government of trying to move past the incident and smooth relations ahead of Yoon's state visit to Washington due later this month.

Dozens of photographs of the documents have been circulating on social media platforms and messaging services including Twitter, Telegram and Discord for at least weeks.

The Pentagon has said it is working to determine if the documents are genuine, and that at least one appeared to have been manipulated.

However, US officials reportedly believe many of the documents are real.

A copy of one of the leaked documents obtained by AFP shows discussions among Seoul's top national security officials about whether providing arms and ammunition to Ukraine would violate the country's long-standing policy against providing weapons to countries in active conflict.

One official suggested exporting ammunition to Poland to get around the policy, the document showed.

The leak has prompted US officials to reassure allies such as South Korea, which has provided non-lethal and humanitarian aid to Ukraine since Russia invaded last year.

Seoul's response to the incident has been critised with former security official Choi Gi-il saying the government is "seemingly defending the US over the suspected surveillance".

"It is very ironic."