A labour rights group said Thursday it had found "deplorable" conditions at Apple suppliers in China, following a probe of several firms that make the US technology giant's hugely popular products.

New York-based China Labor Watch said a four-month investigation of 10 suppliers to Apple in southern and eastern China uncovered violations of workers' rights, including excessive overtime and hazardous work conditions.

"This investigation of 10 different Apple factories in China finds that harmful, damaging work environments characterised by illegally long hours for low levels of pay are widespread in Apple's supply (chain)," it said.

The report was based on surveys and interviews of 620 workers, as well as first-hand observations by a team of six, including some who entered the factories undercover.

The lengthy report followed findings announced in March by the Fair Labor Association (FLA), which toured three Chinese suppliers with Apple's consent and also reported on forced overtime and other problems.

China Labor Watch director Li Qiang urged the California-based company's chief executive Tim Cook to make good on repeated pledges to improve.

"Apple should take the responsibility to change the poor working conditions of those workers," Li told AFP.

Apple did not immediately respond to request for comment on the latest report, but Cook said in February that the company took ensuring good working conditions at its suppliers seriously and was constantly fixing problems.

Following the FLA report, Apple's largest supplier, Taiwan's Foxconn, also pledged to end workplace abuses at its factories in China, including overtime above the amount permitted by Chinese law.

Foxconn has come under scrutiny since 2010, following a spate of suicides and incidents of labour unrest at its Chinese plants.

At least 13 of its employees died in apparent suicides in 2010, with several more deaths last year.

China Labor Watch said other Apple suppliers had treated their staff worse than Foxconn, which has received the most attention.

"The labour rights violations at Foxconn also exist in virtually all other Apple supplier factories and in many cases are actually significantly more dire than Foxconn," the report said.

The labour group found employees worked an average of between 100 and 130 hours of overtime a month at the 10 factories, well above China's legal limit of 36 hours.

Low wages compelled workers to accept overtime and some factories did not properly compensate them for the hours, it said.

Working conditions in factories that produce cases for Apple products were especially poor, including exposure to loud noise and toxic chemicals, the report said.

Workers had little ability to push for better conditions because they did not know how independent unions functioned, it said. China only allows only one national trade union, which has links to the government.

Foxconn responded to the report on Thursday by saying it was committed to making changes following the FLA audit in March.

"The process of change in our company continues, and competitive wages, improved living conditions and the abolition of the use of dispatched workers by our company are some examples of this," it said in a statement.

Another company named in the report, a unit of US-headquartered Jabil Circuit in the southern Chinese city of Shenzhen, denied some of the accusations, including no limits on overtime for some workers.

"Some of the things said by employees do not conform with the company's situation. We abide by state regulations," a personnel official, who declined to be named, told AFP.

Apple products are wildly popular in China, where the iPhone and iPad are particularly coveted by wealthy consumers.

China Labor Watch said more than 70 percent of the workers it surveyed did not own Apple products but would like to have at least one.