bedside in hospital, where family members gathered nearly three weeks after he was admitted with a recurring lung infection.
"Tata (father) is very critical," daughter Makaziwe Mandela told state radio SAFM. "Anything is imminent, but I want to emphasise again that it is only God who knows when the time to go is."
"I won't lie. It doesn't look good," she said. But "if we speak to him he responds and tries to open his eyes -- he's still there."
Clan elder Napilisi Mandela told AFP that the former South African president "is using machines to breathe."
"It is bad, but what can we do," said Napilisi Mandela, who usually presides over family rituals and meetings.
The Nobel Peace Prize winner's health has deteriorated in recent days, prompting Zuma late Wednesday to pull out of a trip to neighbouring Mozambique -- the first time he has scrapped a public engagement since Mandela was hospitalised on June 8.
"President Zuma was briefed by the doctors who are still doing everything they can to ensure his well-being," a statement from the presidency said.
On Thursday a large number of family members gathered at the Mediclinic Heart Hospital, along with the minister of defence, who is responsible for Mandela's treatment. Zuma was also seen returning to the facility.
A few family members walked to the wall of messages and flowers where emotional crowds have been holding vigils, offering their prayers for one of the greatest figures of the 20th century.
With his life seemingly slipping away, messages of support for the former president blanket the wall, including a poster bearing one of his most memorable quotes: "It only seems impossible until it's done".
Supporters sang songs for the father of South African democracy and the architect of remarkable transition from almost half a century of white minority rule to landmark multiracial elections.
Cape Town Archbishop Thabo Makgoba visited the Mediclinic Heart Hospital late Tuesday to pray with wife Graca Machel "at this hard time of watching and waiting".
"Grant Madiba eternal healing and relief from pain and suffering," the prayer said, using Mandela's clan name by which he is fondly known. "Grant him, we pray...a peaceful, perfect, end."
Mandela's lung troubles date from his 27 years locked up on the notorious Robben Island and in other apartheid prisons.
Upon his release he would become the country's first black president.
His "Rainbow Nation" has been forced to come to terms with the increasing frailty of the man who defeated decades of racist white minority rule to become the country's first black president.
Meanwhile messages of goodwill flooded in from overseas.
UN leader Ban Ki-moon said the whole world was praying for "one of the giants of the 20th century".
"I know our thoughts and prayers are with Nelson Mandela, his family and loved ones, all South Africans and people across the world who have been inspired by his remarkable life and example," Ban said.
Hillary Clinton offered "love and prayers to our great friend, Madiba, his family and his nation during this difficult time."
The White House has also sent its wishes but could not yet say whether his ill health would affect a planned visit by US President Barack Obama to South Africa from Friday as part of a tour of Africa.
Mandela's fragile state of health has sparked speculation that the tour could be halted, or radically changed, if the anti-apartheid icon passes away while Obama is on the continent.
South Africa's foreign minister Maite Nkoana Mashabane has said that while Obama would have loved to see Mandela, a meeting with the former South African leader would be impossible.
Mandela -- whose 95th birthday is on July 18 -- has been hospitalised four times since December, mostly for a recurring lung infection dating back to his time in jail for sabotage against the apartheid government.
The world watched as the man once branded a terrorist by the United States and Britain raised his fist as he walked free from prison near Cape Town in 1990.
Mandela went on to negotiate an end to white minority rule and won South Africa's first fully democratic elections in 1994.
He forged a path of racial reconciliation during his single term as president, before taking up a new role as a roving elder statesman and leading AIDS campaigner.
He stepped back from public life in 2004 and has not been seen in public since the football World Cup finals in South Africa in 2010.
With his health now fading, Mandela's supporters are starting to show signs of resignation -- while preparing to celebrate his legacy.
"He's now (nearly) 95 years old so we don't have to worry about him,' 40-year-old Jauffre Basube said. "I think he's done what he's supposed to do."