The deputy head an outlawed Vietnamese Buddhist church, who is under de facto house arrest at a monastery in Ho Chi Minh City, is "critically ill," the church's overseas information arm said.
Thich Quang Do, one of Vietnam's most famous dissidents, has also not been allowed to receive visits from his doctor, the International Buddhist Information Bureau (IBIB) said.
"For the past three days he has been too weak to eat solid food, or even to drink milk. He has not been allowed to receive visits from his doctor," it said in a statement.
The foreign ministry could not be contacted for comment.
Do, 75, is the number two in the Unified Buddhist Church of Vietnam (UBCV), which was banned in 1981 because it refused to come under the control of the Communist Party.
The 2003 Nobel Peace Prize nominee suffers from a heart condition, diabetes and high blood-pressure, and is still recovering from a heart operation he underwent in late August this year, IBIB said.
According to other UBCV monks, Do's health has deteriorated rapidly since he was involved in a 10-hour stand-off with police on October 8 in the central province of Binh Dinh, the IBIB, the church's Paris-based information arm, added.
The following day he and other senior monks were arrested. Do fainted twice during police interrogation and had to be taken to hospital before being taken to the Thanh Minh Zen Monastery in Ho Chi Minh City, it said.
UBCV patriarch Thich Huyen Quang, who has been under effective house arrest without charge or trial for more than two decades, was among those arrested. He was taken back to his monastery in Binh Dinh.
Both Quang and Do, who was released from two years of house arrest in late June, were being investigated because "they violated Vietnamese law," foreign ministry spokesman Le Dung said last week.
The government has accused them of being in possession of state secrets and trying to reorganize the church with the help of outside forces.
It has also said that four other senior UBCV monks involved in the stand-off were placed under house arrest for two years for violating the country's strict national security legislation.
Diplomats and human rights organizations have condemned this recent intensification of the crackdown on the UBCV.