On Wednesday, South Korea's opposition urged the government to investigate alleged US espionage after leaked papers appeared to reveal Washington snooping on its important Asian partner.

A new batch of recently leaked classified US documents suggests that Washington has been eavesdropping on President Yoon Suk Yeol's national security advisors in order to gain arms shipments for Ukraine.

Despite confirming the legitimacy of the documents, defense officials have claimed that some of the files were tampered with, allegedly, to overstate the number of casualties in the war in Ukraine.

South Korea said on Tuesday that specific information contained in a leaked US confidential document that appeared to be based on internal discussions among top South Korean security officials is "untrue" and "altered".

Read more: US scrambles to find source of leak, evaluates impact on national sec

The US spy leak scandal comes just weeks before Yoon is expected to meet with US President Joe Biden in Washington on April 26.

Lee Jae-myung, the head of South Korea's opposition Democratic party expressed Wednesday, "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." 

Former security official Choi Gi-il called it "ironic" that the government was "seemingly defending the US over the suspected surveillance".