BANGKOK (Reuters) -- Myanmar's opposition leader, Aung San Suu Kyi, has been taken to hospital in Yangon with severe diarrhea but her condition is improving, a spokesman for an exiled Myanmar group said on Saturday, citing unnamed sources.

Suu Kyi, who will turn 61 on June 19, was sent to a hospital in the capital on Friday after suffering diarrhea during the previous 24 hours, said Soe Aung, spokesman for the National Council of the Union of Burma (NCUB).

But a spokesman for Suu Kyi's National League for Democracy (NLD) could not confirm reports she was ill, and there was no comment from the military junta.

"We have heard that she was hospitalized yesterday afternoon after she had diarrhea 30 times starting from the day before yesterday," Soe Aung said in a phone interview from his house near the Thai-Myanmar border.

"We don't know the cause but she was allowed to be admitted to a hospital and has been recovering since yesterday evening," he said, citing unnamed "reliable sources" in Yangon.

A U.S. State Department official voiced concern on Friday over reports that the Nobel laureate might be in hospital and urged the Myanmar authorities to release her from house arrest.

"We would call upon the Burmese (Myanmar) government to provide Aung San Suu Kyi any and all medical assistance that she might need and to do so expeditiously and to ensure her safety during any treatment," said State Department spokesman Sean McCormack.

Myanmar confirmed on Friday that Suu Kyi's house arrest had been extended for one year.

Suu Kyi, who won the Nobel Peace Prize in 1991, has been in prison or under house arrest for 10 of the past 17 years. Her party won landslide elections in 1990 but the military refused to relinquish power.

"We are very much concerned that a person of her age had this kind of serious diarrhea. We are very concerned about her well-being," Soe Aung said.