Hwang Woo Suk, the leading stem cell expert whose South Korean team cloned the first human embryo and created the first cloned dog, publicly apologized Thursday for ethical breaches at his lab and said he would resign from all his official posts.

Under mounting pressure from the international scientific community, Hwang, 52, admitted that his team had used ova samples extracted from two of his junior scientists during research that led to the team's historic cloning of a human embryo in 2003.

Such practices are considered highly unethical in international scientific circles. The practice of obtaining eggs from female team members is widely viewed as off-limits because of the potential for subtle coercion, given the hierarchal structure of lab research -- something especially true in South Korea.

Choking back tears, Hwang said that he had not known about the women's donations until the magazine Nature began investigating the source of his team's ova specimens early last year.

Even after discovering the truth, however, he denied the allegations out of fear his project would be jeopardized, he said. The women, he said, had asked that their privacy be maintained. "Being too focused on scientific development, I may not have seen all the ethical issues related to my research," Hwang said.

A national hero in South Korea, Hwang has recently been dogged by international allegations of ethical impropriety that have threatened to severely set back his work. Last month, Hwang, along with several other leading scientists, launched the Seoul-based World Stem Cell Hub, a project to find treatments for diseases that remain incurable.

But earlier this month, University of Pittsburgh researcher Gerald Schatten said he was pulling out of his association with Hwang, citing concerns about the way the group had obtained human eggs -- whose difficult procurement is typically one of the most vexing obstacles to large-scale stem cell research.

The controversy underscored how difficult it will be to hold to internationally accepted standards in the highly sensitive field of stem cell research and cloning -- particularly when different societal traditions are involved. Thousands of South Koreans took to the Internet and airwaves this week to defend Hwang, with many appearing baffled by why foreigners would see the donations as ethical violations.

"Hwang had no Western educational background, and unlike the West, Korean institutions have no systematic way of reviewing, judging and clarifying research results in a step-by-step process," said Hwang Yoo Sung, president of NeoDin Medical Institute, a Seoul-based research lab. He is not related to Hwang Woo Suk. "All researchers know that they need to abide by certain ethical procedures when they launch a project, but in reality, researchers in South Korea don't feel the need to constantly contemplate these issues."

The two women, in statements to a government commission that were later made available to the news media, said they had made their donations in secret and under false names after Hwang had refused their offers. At the time, the team was desperately in need of additional ova for their stem cell work.

In an interview with The Washington Post, Hwang said Thursday that he had turned the women down to avoid pressuring other female staffers to do the same. He said he confronted them in May 2004, after a reporter from Nature magazine made inquiries about the donations. One of the women, a PhD student researcher, admitted to the reporter that she had made a donation, then retracted the statement. Hwang said that soon after, both women told him, but said neither he nor the women clearly "realized it was ethically wrong at the time."

After Nature's article stirred the scientific community, Hwang said, he came to understand the implications. But he withheld the truth, he said, to protect his researchers from public embarrassment.

This year, Hwang's team was again hailed for creating Snuppy, the world's first cloned dog. But the ethical allegations persisted. And on Monday, Roh Sung Il, head of Seoul-based MizMedi Women's Hospital, also admitted that he had paid about $1,447 each to 20 women to gather human eggs for Hwang's research.

Roh said that he had not informed Hwang about the payments until recently. The payments, which ended in 2003, were not illegal under existing South Korean laws.

An investigation conducted by South Korea's Health Ministry, whose findings were also released on Thursday, described the donations by Hwang's team members as not "coerced or coaxed" and concluded that there had been "no violation of ethics guidelines" by Hwang's team.

But Hwang had always insisted that the eggs used in his research were from donors who gave them in hopes of helping his work, and not for profit.

He took full responsibility for his mistakes and issued an emotional apology to the nation. Though he will continue with his research, he will step down as head of the World Stem Cell Hub as well as other government-funded scientific groups with which he is now associated.

One of his biggest problems, Hwang said, was that he was setting precedents. "Everything we tried was the world's first," he said. "We were making the very first footprints on a snow-covered field. In that process, we lacked insight into law and ethics. Scientific research should be conducted within the boundaries of ethics, but in reality, there were some cases in which the ethics regulations lag behind development of science."

The South Korean government, which has staunchly backed Hwang, announced the creation Thursday of a new state-run ova bank with firm, closely monitored guidelines. Such a move would mark the first time a nation has taken such an official role in stem cell research, highlighting the scramble by South Korean officials to salvage their country's reputation as the world's leading center for such science.

As of Thursday, 188 women had already officially pledged to donate their eggs to a network organized by a group of South Korean celebrities, politicians and citizen groups set up last week to help Hwang's team.

Cho reported from Seoul.

Hwang Woo Suk said he would resign from all his official posts.