ROME -- The World Health Organization says China has taken steps to end its once-widespread practice of harvesting organs from executed prisoners but that it's impossible to know what is happening across the entire country.

At a Vatican conference on organ trafficking this week, a former top Chinese official said the country had stopped its unethical program, but critics remain unconvinced.

In an interview Thursday, WHO's Jose Ramon Nunez Pena said he personally visited about 20 hospitals in China last year and believes the country has reformed. But he acknowledged that it was still possible "there may still be hidden things going on." China has more than 1 million medical centers, although only 169 are authorized to do transplants.

Nunez Pena said he had seen data including organ transplant registries and was convinced the country was now shifting away from illegally harvesting organs.

"What is clear to me is that they're changing," he said. "But in a country as huge as China, we can't know everything."

Earlier this week, critics questioned China's claims of reform and suggested that WHO should be allowed to conduct surprise investigations and interview donor relatives. The U.N. health agency has no authority to enter countries without their permission.

China's Dr. Haibo Wang responded that China shouldn't be singled out for such treatment while other countries were not. The head of the Chinese delegation, Dr. Huang Jiefu, told the conference there had been an increase in both living and deceased voluntary organ donors following China's crackdown on the illicit organ trade.

"It sounds a little hard to believe that China could have so quickly made this change to its organ donation program," said Vivek Jha, executive director of the George Institute for Global Health in India.

He said China should provide the international transplant community with data to prove that its organs are no longer being illegally procured.

"It could be the case that China has changed," he said. "The problem is we just have not seen the information to prove it."

Nunez Pena said tracking illegal organ activities was inherently difficult and that countries with past problems like India and Costa Rica appeared to have improved practices, but that officials couldn't be absolutely certain that was the case. He said WHO officials were now focusing on other countries like Egypt and Sri Lanka as worrisome centers of organ harvesting.

Campbell Fraser, an organ trafficking researcher at Griffith University in Australia, agreed the trends over the past few years have shown a drop in the number of foreigners going to China for transplants and an increase of organ seekers heading to the Middle East.

At a press conference at the Chinese Embassy in Italy following the two-day Vatican organ conference, Fraser said migrants - including Syrians, Somalis and Eritreans - sometimes resort to selling off a kidney to pay traffickers to get them or their families to Europe.

"Egypt is where the biggest problem is at the moment," he said, adding that it has the best medical facilities in the region and can perform the live donor surgeries.

He estimated as many as 10 such illicit transplants could be happening per week, though he had no statistics and said he based his research largely on anecdotal information from recipients, law enforcement, doctors and even some organ "brokers."

Fraser said he has access to transplant patient "chat boards" because he himself had a kidney transplant in his native Australia in 2003.

Nunez Pena said it was likely that organ trafficking would find its way to conflict-plagued regions.

"We're hearing about a lot of problems in Egypt, Pakistan and the Philippines," he said, predicting that authorities were poised to break up an organ smuggling ring in Egypt in the next few weeks. "Wherever you have vulnerable people, you will see these kinds of problems."