Apple admitted Friday some of its suppliers continued to overwork and underpay employees, as it threw open its factory doors to monitors after a spate of suicides at a Chinese plant.

After fielding heavy criticism for not policing conditions for the workers who produce its popular iPads, iPhones and other products, Apple divulged its list of suppliers and said it would allow an outside labor rights group to monitor them.

Meanwhile an annual internal audit of its supply chain showed that just 38 percent of the company's suppliers adhered to Apple's own standard of a maximum 60 hour work week and minimum one day off per week.

One-third of its suppliers were negligent in managing hazardous substances, and the same portion were below standard in injury prevention practice.

The company said it believed it had eliminated child labor among its final assembly suppliers.

"We will continue regular audits and go even deeper into our supply chain to ensure that there are no underage workers at any Apple supplier," it said in the report.

It also said it had forced suppliers last year to reimburse workers for some $3.3 million they paid in excess broker fees to get jobs at Apple supplier plants. Apple said it had forced a total of $6.7 million in such reimbursements since 2008.

"To the best of our knowledge, Apple is the only company in the electronics industry that mandates reimbursement of excessive recruitment fees."

The moves came after Apple endured heavy criticism for working conditions at some suppliers, and especially after a spate of suicides at a Chinese factory producing the iPhone that is owned by Taiwan's Foxconn group.

Steve Jobs, Apple's co-founder who died in August, was forced to deny in 2010 that the company supported worker-abusing "sweatshops."

Apple said its newest Supplier Responsibility report involved 229 audits throughout the company's supply chain last year, 80 percent more than in 2010, the year criticism of the company from labor rights groups intensified.

In parallel with the release of the report, Apple said it would allow the Fair Labor Association to independently investigate and report on conditions in its suppliers.

"With the benefit of the FLA's experience and expertise, we will continue to drive improvements for workers and provide even greater transparency into our supply chain," said Jeff Williams, Apple's senior vice president of operations.

Meanwhile Apple also divulged for the first time a list of 156 suppliers who provide 97 percent of its materials and manufacturing services by value.

The company has long kept the list secret, adding to the criticisms from groups saying it does not work hard enough to protect workers making its products.

The list is a Who's Who of global tech suppliers, including many from Japan, China, and South Korea.

Other US companies like Hewlett-Packard and Nike have released their supplier lists to help counter similar criticism.