SEOUL, April 11 (Reuters) - A senior South Korean security official said on Tuesday that information contained in purportedly leaked U.S. confidential documents that appeared to be based on internal discussions among top South Korean officials is "untrue" and "altered."

Deputy National Security Adviser Kim Tae-hyo made the remark as he departed for Washington ahead of President Yoon Suk Yeol's state visit to the U.S. on April 26, stressing that the two countries' alliance remained strong.

Several documents have recently been posted on social media offering a partial, month-old snapshot of the war in Ukraine, including one that gives details of internal discussions among South Korean officials about U.S. pressure on Seoul to help supply weapons to Ukraine.

"The two countries have a same assessment that much of the information disclosed is altered," Kim told reporters, adding that the report on South Korea is "untrue."

He did not elaborate which part of the document was untrue.

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 U.S. replenish its stockpiles, insisting that the "end user" should be the U.S. military. But internally, top South Korean officials were worried that the U.S. 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.

The U.S. report appeared to be based in part on signals intelligence, which suggests the United States had been spying on South Korea, one of its most important allies.

Reuters has not independently verified the authenticity of the documents. Some giving battlefield casualty estimates from Ukraine appeared to have been altered to minimise Russian losses.

Kim said the latest revelation will not have an impact on South Korea's alliance with the U.S.

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

Earlier, U.S. Secretary of Defense Lloyd Austin held phone talks with his South Korean counterpart on Tuesday and discussed recent media reports on the leak of confidential U.S. documents, South Korea's defence ministry said.

During the phone conversation, which took place at the request of Austin, the Pentagon chief vowed to closely communicate and cooperate with South Korea on the issue, the ministry said.