BANGKOK, Oct 10 (Reuters) - A Myanmar opposition leader who was arrested during last month's mass protests against the junta died due to torture during interrogation, an activist group said on Wednesday.

In Washington, the United States threatened new sanctions against Myanmar after media reports of the death of Win Shwe.

"The junta must stop the brutal treatment of its people and peacefully transition to democracy or face new sanctions from the United States," White House spokesman Gordon Johndroe said in a statement.

The White House did not say what additional sanctions it was considering on the former Burma, but it called for a full investigation into Win Shwe's death.

The Assistance Association for Political Prisoners (AAPP) said that Win Shwe, a 42-year-old member of the National League for Democracy, and four other people were arrested on Sept. 26 because of their active support for and participation in the biggest pro-democracy protests in nearly 20 years.

"He died as a result of torture during interrogation," the Thai-based group said in a statement on its Web site (www.aappb.org), sourcing its information to authorities in Kyaukpandawn township.

"However, his body was not sent to his family and the interrogators indicated that they had cremated it instead."

Official media in Myanmar said 10 people were killed when the junta sent in soldiers to end days of Buddhist monk-led demonstrations in September, although Western governments say the toll is likely to have been much higher.

The AAPP said in its statement that "many dead bodies and injured persons were cremated or placed in the river".

"Some dead bodies of monks have appeared in the Pazundaung River in Rangoon (Yangon) in the past few days. In addition, many of those who have been arrested have been tortured during interrogation."

U.S. First Lady Laura Bush told USA Today in an interview published on Wednesday that the United States would announce further sanctions on Myanmar's military government "within the next couple of days" if the junta does not take steps toward democracy.

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