The work of the world's foremost human cloning researcher, Hwang Woo-suk , has been thrown under an ethical cloud, jeopardizing the bold international cloning project that he and several prominent U.S. researchers announced just last month.

The World Stem Cell Hub foundation had announced plans to open cloning centers in San Francisco and London. But U.S. support for the effort is now waning after Hwang was accused of obtaining egg donations from a subordinate and misleading a U.S. collaborator about it.

The scientific dustup is also renewing debate over the thorny issue of how scientists plan to collect women's eggs vital to their controversial work. Thousands of eggs are necessary to complete cloning projects, and few ethical guidelines exist governing how donors should be treated.

The San Francisco-based Pacific Fertility Center, which had said it would help the stem cell hub collect eggs beginning in January, said on Monday that it has severed all ties with Hwang and has dropped all involvement with cloning research.

Clinic spokesman Scott Kaplan declined further comment.

Meanwhile, the nonprofit Children's Neurobiological Solutions Foundation said it was putting on hold a grant application from the Korean-led stem cell hub.

"These are very serious claims being made," said Shane Smith, science director of the Santa Barbara nonprofit, which seeks treatments for childhood brain disorders.

Smith declined to give the amount of the grant request, but said it exceeded the small nonprofit's usual maximum of $75,000.

University of Pittsburgh cloning researcher Gerald Schatten said on Saturday that he resigned from the stem cell hub and ended his 20-month collaboration with Hwang because of the South Korean's "unethical practices" in collecting eggs from a volunteer, then misleading Schatten about it.

Schatten released a statement on Saturday announcing his resignation from the stem cell hub and has declined further comment.

Last year, Hwang's team at Seoul National University became the first to successfully clone a human embryo. Since then, though, rumors have swirled that some of the 242 eggs used in the experiment were donated by subordinate scientists in Hwang's famed cloning lab.

Scientists and ethicists said Monday that collecting eggs from an employee is unethical because of the potential for subordinates to feel coerced.

Hwang has steadfastly denied those accusations and again defended his research on Monday in Seoul.

"All research up until now has been conducted in strict observance of the government-set guidelines," Hwang said, according to South Korea's Yonhap news agency. He didn't elaborate, saying he would "divulge everything" at an appropriate time.

Nonetheless, Schatten's accusation has cast a dark cloud over Hwang's work and is raising anew one of the thorniest ethical dilemmas cloning researchers face in collecting eggs for their work.

There are no known human cloning project ongoing in the United States, though Harvard University researchers have asked school officials for permission and the $3 billion California Institute for Regenerative Medicine said it would fund such work.

Stem cell scientists hope to clone embryos to extract stem cells in order to watch how diseases develop and create new drugs.

The basic idea of cloning is to take a patient's genetic material and plop it into an unfertilized human egg. The implanted DNA then drives the egg to develop into an embryo.

The problem is how to obtain the eggs, especially considering how inefficient cloning technology is. South Korean researchers in 2004 used 242 eggs from 16 donors to yield just one cloned human embryo, which was destroyed after 14 days.

About 100,000 American women are injected annually with hormones to stimulate their ovaries to "superovulate" each year at fertility clinics in attempts to conceive babies. The process is arduous, and there's a 1-in-50 chance a patient will over-respond to the hormones, causing complications

Critics of collecting eggs for research contend that there may be long-term health consequences of fertility drugs and that a small percentage of women who go through the process may suffer adverse effects.

The Bedford Stem Cell Research Foundation in Somerville, Mass. is the only known facility to have collected eggs purely for research. It has paid about 20 women about $4,000 each plus expenses to take fertility hormones but hasn't been active since the first of the year as it struggles financially and is reworking its own ethical guidelines.

"It all comes down to informed consent," said foundation director Ann Kiessling, who said she declined an invitation to work with the South Korea-led stem cell hub.

Two leading stem cell teams at Stanford University and the University of California, San Francisco, as well the California Institute of Regenerative Medicine, also declined similar invitations to work with the stem cell hub.