South Korean cloning expert Hwang Woo-suk has stood by his apparently breakthrough research, despite claims some of the results were fabricated.
Dr Hwang has been hailed as a hero in South Korea
Dr Hwang said his work would be vindicated within 10 days, after stem cells from his study were reassessed.
But he has asked the journal Science to retract a key research paper, published in June, due to the controversy.
The cloning pioneer was speaking on Friday, a day after a colleague claimed Dr Hwang had admitted faking some data.
Even before the latest claim, Dr Hwang was already under fire after admitting ethical lapses in his research.
The controversy is causing shockwaves in South Korea, where the cloning pioneer is widely considered a national hero.
After an emergency meeting on Friday in response to the fabrication claims, the government said it would wait to take further action until after an investigation by Seoul National University, where Dr Hwang works.
Dr Hwang resigned from his main post as head of the World Stem Cell Hub in November, after it emerged that some of the eggs used in his research were donated by his staff - in contravention of international guidelines.
Now it is some of the research itself which is being called into question.
Hwang Woo-suk successfully cloned a puppy, called Snuppy
The latest allegations came from a colleague, Roh Sung-il, who collaborated with Dr Hwang on a landmark research paper, which was published this year in the magazine Science.
After visiting Dr Hwang in hospital, where he was being treated for stress, Mr Roh told South Korea's MBC television network that Dr Hwang had admitted fabricating some of his key research results.
In a press conference on Friday, Dr Hwang acknowledged that some of the stem cells created and referred to in the article had died after being contaminated.
But he said other cells were now being unfrozen, and would serve to prove the authenticity of his work.
"Our research team made patient-specific embryonic stem cells and we have the technology to produce them," Dr Hwang said.
He said he was retracting the paper from Science because of the controversy, but he emphasised that he did not doubt his findings.
According to Reuters news agency, he said a follow-up paper had been submitted to another journal which would restore faith in his team.
Dr Hwang's paper claimed to have produced 11 stem cell lines from the skin of people with disease or injuries.
But Mr Roh said on Thursday that a scientist working in Dr Hwang's laboratory was pressured into doctoring results.
Dr Hwang said on Friday that he was "shocked" by Mr Roh's remarks. "Our six research members made 11 stem cells and all confirmed this," he said.
But he admitted that the cells had been badly contaminated by a fungus, and said he planned to ask prosecutors to investigate his suspicion that they may have been tampered with or replaced.
Research into stem cells is being keenly watched because of its possible benefit to sufferers of illnesses such as Alzheimer's and Parkinson's.
Stem cells can grow into any type of cell, so researchers hope to harvest them and use them to replace ones that have become worn out or damaged.
Dr Hwang's other research has not been questioned. He claimed last year to have cloned the world's first human embryos and to have extracted stem cells from them, as well as producing the world's first cloned dog earlier this year.