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

Several documents have been recently posted on social media offering a partial, month-old snapshot of the war in Ukraine, sparking a diplomatic row between the US and some allies.

One of the documents gave details of internal discussions among South Korean officials about US pressure on Seoul to help supply weapons to Ukraine, suggesting the US could have been spying on South Korea, one of its most important allies, and inviting condemnation from the Asian nation's lawmakers.

The office of South Korean President Yoon Suk Yeol said in a statement that suspicions his office in Seoul was monitored are "utterly false” and that any attempts to shake its alliance with the US is an act“compromising national interest”.

US Secretary of Defense Lloyd Austin held phone talks with his South Korean counterpart on Tuesday during which the two sides agreed that much of the document on South Korea has been fabricated, Yoon's office said.

It did not elaborate on which part of the document was untrue. South Korea's defence ministry said that during the phone conversation that took place at the request of Austin, the Pentagon chief explained about recent media reports on the leak and vowed to closely communicate with South Korea on the issue.

The revelation comes just weeks before Yoon is scheduled to meet with US President Joe Biden in Washington, on April 26. Some lawmakers of South Korea's main opposition Democratic Party expressed “strong regret” on Monday over the alleged surveillance, calling it a clear violation of national sovereignty and a major security failure of the Yoon administration.

Kim Tae-hyo, South Korea's Deputy National Security Adviser, said the latest controversy will not have an impact on South Korea's alliance with the US, as he departed for Washington ahead of Yoon's visit.

"The US is the country with the world's best intelligence capabilities and since (Yoon's) inauguration we have shared intelligence in almost every sector,” Kim told reporters.

The document, which does not appear to have a date on it, said that South Korea had agreed to sell artillery shells to help the US replenish its stockpiles, insisting that the “end user” should be the US military. But internally, top South Korean officials were worried that the US would divert them to Ukraine.

South Korea has said its law forbids supplying weapons to countries engaged in conflict, meaning it can't send arms to Ukraine. Reuters has not independently verified the documents' authenticity. US officials have said some giving battlefield casualty estimates from Ukraine appeared to have been altered to understate Russian losses.