The 63-year-old Nobel laureate, whose detention comes up for renewal late this month, has low blood pressure, is dehydrated and has been placed on an intravenous drip by her doctor's assistant.
Official sources said on Thursday that Ms Suu Kyi's doctor, Mr Tin Myo Win, had been denied permission to enter her lakeside prison home. Mr Nyan Win said he did not know the reason for this action.
The detention adds more mystery to the alleged visit last Sunday by an American who swam across a lake to her home.
State media said the man, identified as Mr John William Yeattaw, 53, spent two days at the house before security forces plucked him from the water as he left at dawn on Wednesday.
He was reportedly arrested with a large empty water bottle - which might have been used as a flotation device.
The man's motive remained unclear. Official sources have said he is a Vietnam war veteran, while Ms Suu Kyi's lawyer Kyi Win said he is an 'adventurous' American acting of his own accord.
Ms Suu Kyi, who has been isolated and under house arrest for most of the past 19 years, is allowed virtually no visitors, aside from her doctor.
The authorities have not said if they intend to extend her detention. But Western diplomats say she is unlikely to be released.