Dr. Kevin Loh, chief executive of Mount Elizabeth Hospital, said the woman was in "extremely critical condition," The New York Times reported.
"Our medical team's investigations upon her arrival at the hospital yesterday [Thursday] showed that in addition to her prior cardiac arrest, she also had infection of her lungs and abdomen, as well as significant brain injury," Loh said in a statement. "The patient is currently struggling against the odds, and fighting for her life."
The woman was transferred to the Singapore hospital after she was initially treated in a hospital in a New Delhi. The attack occurred in New Delhi Dec. 16 and triggered violent protests that left one police officer dead.
Six men were arrested and two police officers were suspended following the attack.
Dr. Mahesh Chandra Misra, head of the department of surgical disciplines at the All India Institute of Medical Sciences and who was a member of the team caring for the patient in New Delhi, told the Times her injuries were the worst he'd ever seen.
"As doctors, we've never witnessed anything like this," he said.
The patient was "practically dead" when she arrived at the hospital and had to be resuscitated, Misra said. Her small and large intestines were removed because they had become gangrenous.
Police said the victim and a friend were returning from a movie when they boarded a bus where she was gang-raped for nearly an hour, the BBC reported. Police said the woman and her friend also were beaten with iron bars and thrown from the bus as it was moving.
Doctors said the woman's family traveled with her to Singapore.
Indian Home Minister Sushil Kumar Shinde said in a statement the government sent the victim to the Singapore hospital on the recommendation of her doctors, the BBC said.
"Despite the best efforts of our doctors, the victim continues to be critical and her fluctuating health remains a big cause of concern to all of us," he said.
The government has tried to address public ire over the incident by announcing a series of measures meant to make conditions in New Delhi safer, including additional night patrols, checks on bus drivers and their assistants, and banning buses with tinted windows or curtains, the BBC said. The government also said it will post photos, names and addresses of convicted rapists on official websites.