The 23-year-old arrived in Singapore on Thursday after undergoing three operations in a Delhi hospital.
“Her vital signs are deteriorating with signs of severe organ failure,” hospital official Kelvin Loh said.
The attack earlier this month triggered violent public protests in India that left one police officer dead.
Six men have been arrested and two police officers have been suspended following the 16 December attack.
Doctors had earlier described the woman as “fighting for her life”.
Kelvin Loh said in a statement that the hospital had put her on “maximum artificial ventilation support, optimal antibiotic doses as well as stimulants which maximize her body’s capability to fight infections”.
“Her family members have been informed that her condition has deteriorated and they are currently by her side to encourage and comfort her,” he said.
“The High Commission of India is with her and her family at this critical time. Our medical team continues to provide all possible treatment and care,” Kelvin Loh added.
On arrival at the hospital in Singapore, doctors said that as well as a “prior cardiac arrest, she also had infection of her lungs and abdomen, as well as significant brain injury”.
The government has tried to halt rising public anger by announcing a series of measures intended to make Delhi safer for women.
These include more police night patrols, checks on bus drivers and their assistants, and the banning of buses with tinted windows or curtains.
The government has also said that it will post the photos, names and addresses of convicted rapists on official websites to shame them.
The victim and her friend had been to see a film when they boarded the bus in the Munirka area, intending to travel to Dwarka in south-west Delhi.
Police said she was raped for nearly an hour, and both she and her companion were beaten with iron bars and thrown out of the moving bus into a Delhi street.
India’s Home Minister Sushil Kumar Shinde said in a statement on Thursday that the government had decided to send the victim overseas on the recommendation of her doctors.
“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.
Image source: Wikimedia Commons President-elect Donald Trump celebrated his election victory at the Ultimate Fighting…
Millions of voters across the US chose to return Donald Trump to the White House…
Donald Trump declares victory in the US election as he addresses jubilant supporters in Florida.…
Stocks around the world are rising as Donald Trump appears to be on the cusp…
Donald Trump has won Pennsylvania, North Carolina and Georgia and taken a lead over Kamala…
Quincy Jones, the celebrated musician and producer who worked with Michael Jackson, Frank Sinatra, Ray…