WASHINGTON — Leaked Pentagon documents appear to reveal a sensitive conversation between high-level South Korean officials on whether to send weapons to Ukraine.

The secret document, seen by the BBC, suggests that the US has been spying on its decades-old ally South Korea.

It was in a leak that includes information about the war in Ukraine, as well as on China and US allies.

The leak has the potential to upset South Korea's relationship with both the US and Russia.

South Korea says it is investigating the leak but has insisted that it is impossible to intercept private conversations inside its presidential office, and that this discussion could not have taken place in its private underground bunker.

Washington has been scrambling to trace the source of the leaks, which the Pentagon says is a serious risk to national security.

The document seen by the BBC appears to show that South Korea was torn over whether to sell ammunition that could be used by Ukraine.

Washington has been pushing Seoul to arm Kyiv, but it has so far resisted, citing its policy not to supply weapons to countries at war.

Last year, South Korea agreed it would sell artillery shells to the US, to replenish its stocks, which have been depleted by the war in Ukraine.

As part of the deal, Seoul insisted that the US had to keep the shells for itself, they could not be diverted onto Kyiv. The leaked report shows the government was concerned about the deal and suspected the US might indeed pass the artillery on to Ukraine.

It details a sensitive, high-level conversation from 1 March 2023 between two of South Korean President Yoon Suk Yeol's senior national security advisors.

President Yoon's foreign affairs secretary Yi Mun-hui reportedly told the then National Security Advisor Kim Sung-han that the government was "mired in concerns that the US would not be the end user" of the ammunition.

They also worried that President Biden could call President Yoon directly about the issue, and that if South Korea were to change its policy on providing weapons to Ukraine, it could look as if it had been pressured by the US.

According to the document, South Korea's national security advisor, Mr Kim, then suggested they could sell shells to Poland instead, given that "getting the ammunition to Ukraine quickly was the ultimate goal of the United States".

The US has made no secret of the fact that it wants Seoul to arm Ukraine. It believes South Korea, with its ability to build advanced weapons at a breakneck speed, could make a significant contribution to the outcome of the war.

But Seoul has been reluctant to do so, for fear of burning bridges with Russia. This leak suggests Seoul not only understood that South Korean shells could end up in Ukraine, but that they were open to this happening, which could strain its relationship with Russia.

"South Korea always plays this delicate balancing act, with the US on one side, and Russia and China on the other," said Jenny Town, a Korea analyst from the think tank 38 North. "This leak shows it is the optics they are most concerned about. They're trying to balance what they're willing to do to support Ukraine with how it will be perceived."

The timing of the leak is unfortunate. In a fortnight President Yoon will travel to the White House on a state visit to celebrate 70 years of the alliance between the two countries - an alliance the US is at pains to point out is still "iron-clad".

This has triggered security concerns in Seoul, with the opposition party questioning how the US was able to intercept such a high-level conversation. "This is a clear violation of our sovereignty by the United States and a super-scale security breach on the South Korean part," it said in a statement on Monday.

Kim Jong-dae, an advisor for the former liberal government, describes this as an "intelligence disaster" for the South Koreans. "This is the tip of the iceberg. There is no way in hell this is it," he said.

South Korea's government is trying to downplay the leak. It says it agrees with a US assessment that some of the documents may have been distorted.

Meanwhile a government source warned that any attempt to "exaggerate or distort this incident, to shake the alliance ahead of the summit, will be resisted".

The US was expected to use the upcoming summit to further press Yoon to send weapons to Ukraine. That matter is suddenly more delicate. — BBC