"I urge the Egyptian government to immediately release Abdel Fattah from prison and provide him with the necessary medical treatment," Turk said in a statement, warning that the activist "is in great danger."
"His dry hunger strike puts his life at acute risk."
Abdel Fattah has since late last year been serving a five-year sentence for "broadcasting false news", having already spent much of the past decade behind bars.
After a seven-month hunger strike during which he consumed only "100 calories a day", he has for the past week refused food altogether, and on Sunday he stopped drinking water to coincide with the opening of the COP27 climate summit in Egypt.
"We're very concerned for his health," UN rights office spokeswoman Ravina Shamdasani told reporters in Geneva, also lamenting "a lack of transparency as well around his current condition."
She said Turk had raised Abdel Fattah's case with Egyptian authorities on Friday.
UN Secretary-General Antonio Guterres had also done so on the COP27 sidelines, UN spokeswoman Alessandra Vellucci told reporters.
Abdel Fattah's case has sparked an outcry at the COP27 Climate Summit being held at Egypt's Sharm El Sheikh resort.
British Prime Minister Rishi Sunak and French President Emmanuel Macron both met directly with Egyptian President Abdel Fattah al-Sisi on Monday and upped the pressure for his release, hours after three Egyptian journalists said they had begun their own hunger strikes over his fate.
Activists at COP27 have also been posting prolifically on Twitter under the hashtag #FreeAlaa, and several speakers have ended their speeches with the words "you have not yet been defeated" -- the title of his book.
According to rights groups, Abdel Fattah is among more than 60,000 prisoners of conscience in Egypt since Sisi came to power after deposing Islamist president Mohamed Morsi in 2013.
The UN rights chief noted that the resumption in April 2022 of Egypt's Presidential Pardon Committee "had resulted in numerous individuals being released".
But he called "on the Egyptian authorities to fulfil their human rights obligations and immediately release all those arbitrarily detained, including those in pre-trial detention, as well as those unfairly convicted."
"No one should be detained for exercising their basic human rights or defending those of others."