A leaked Pentagon document seen by the BBC reveals a sensitive conversation between high-level South Korean officials on whether to sell weapons that could be used in Ukraine.

The intercept targeted two of President Yoon Suk Yeol's senior advisors.

They are torn between US pressure to send ammunition to Ukraine and their policy not to arm countries at war.

One of the advisers suggests sending the shells to Poland instead, to avoid appearing to have given in to the US.

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

Last year, South Korea agreed it would sell artillery shells to the US to replenish its stocks, but insisted the US had to keep the shells for itself and not send them on to Kyiv.

In their conversation on 1 March, 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".

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.

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, repeatedly citing its policy of not sending weapons to countries at war, while privately worrying about burning bridges with Russia.

This leak suggests that the South Koreans not only understood that their shells could end up in Ukraine, but that they were open to this happening. This could strain its relationship with Moscow.

"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."

This report, seemingly based on signals intelligence, also has the potential to upset US-South Korea relations, as it suggests the US has been spying on its decades-long ally.

Although not a surprise that the US spies on its friends and enemies alike, the timing of this disclosure 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".

The report 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 Mr Yoon to send weapons to Ukraine. That matter is suddenly more delicate.