Apple, following reports of harsh working conditions, said the watchdog Fair Labor Association began inspections at a Foxconn plant in China that makes products for the California gadget-maker.
"We believe that workers everywhere have the right to a safe and fair work environment, which is why we've asked the FLA to independently assess the performance of our largest suppliers," Apple chief executive Tim Cook said.
"The inspections now underway are unprecedented in the electronics industry, both in scale and scope," said Cook, who took over as chief executive last year from Apple's late co-founder Steve Jobs.
Apple agreed last month to allow inspections by the independent labour watchdog association following reports that employees were overworked and underpaid at Foxconn factories in China.
The Taiwan-owned Foxconn is the largest manufacturer of Apple products.
Apple said the first inspections began Monday morning at a facility in Shenzhen known as Foxconn City. They were carried out by a team of labour experts led by FLA president Auret van Heerden.
Apple said the FLA will "interview thousands of employees about working and living conditions including health and safety, compensation, working hours and communication with management."
"The FLA's team will inspect manufacturing areas, dormitories and other facilities, and will conduct an extensive review of documents related to procedures at all stages of employment," the company said.
The Cupertino, California-based Apple said its suppliers have pledged "full cooperation with the FLA, offering unrestricted access to their operations."
Apple said the FLA's findings and recommendations will be posted on its website, fairlabor.org, in early March.
Besides Foxconn plants, FLA teams will also inspect factories owned by two other Taiwan-owned manufacturers, Quanta and Pegatron, which also make Apple products.
"When completed, the FLA's assessment will cover facilities where more than 90 per cent of Apple products are assembled," Apple said.
Apple joined the FLA last month and divulged for the first time a list of its suppliers.
Last week, petitions were delivered to Apple stores in several countries denouncing working conditions at Chinese factories making Apple gadgets.
In 2010, Apple was accused of abetting poor worker conditions after a rash of worker suicides at a Foxconn plant in China.
The New York Times reported last month that workers assembling iPhones, iPads and other devices at Foxconn facilities in China "often labour in harsh conditions" and work "excessive overtime."
According to the newspaper, two explosions at iPad factories last year killed four people and injured 77.
The recent criticism of working conditions in China has done little to dent Apple sales and shares of the company surged past US$500 for the first time on Wall Street yesterday.
Apple reported blockbuster quarterly earnings last month with net profit more than doubling to a record $13.06 billion and revenue soaring to an all-time high of $46.33 billion.
Shares of Apple have risen steadily over the past few years on the release of a string of hit products starting with the iPod in 2001, followed by the iPhone in 2007 and the iPad in 2010.