Apple CEO Tim Cook came out swinging today in response to a BBC report that alleged workers in Chinese iPhone factories were mistreated and subject to numerous labor violations.

In a piece aired on the BBC’s "Panorama," undercover reports found violations of working standards at Pegatron factories, including extended working hours, confiscated ID cards, cramped dormitories and coached safety exams. But Apple disagrees with the documentary’s conclusion.

Apple’s CEO was "deeply offended by the suggestion that Apple would break a promise to the workers in our supply chain or mislead our customers in any way,” according to a letter to employees from Jeff Williams, Apple senior vp of operations. “Panorama’s report implied that Apple isn’t improving working conditions. Let me tell you, nothing could be further from the truth," said the letter, excerpts of which were published by the Telegraph.

The BBC’s documentary alleged that workers routinely fell asleep on 12 hours shifts, but some worked as long as 16 hours a day. Others were made to work 18 days straight.

Apple declined to be interviewed for documentary, but it provided the BBC with a statement: "We are aware of no other company doing as much as Apple to ensure fair and safe working conditions.”

The company claims it limits workweeks to 60 hours a week “except in unusual circumstances,” according its latest supplier responsibility report. Among employees who worked over 40 hours a week in 2013, the average number of hours worked was 54.

Despite Apple’s claims that overtime should be voluntary, undercover reporters routinely worked overtime hours without a choice, the BBC documentary alleged. Pegatron is reportedly investigating the allegations of the documentary and claims it would take action to correct any issues found.

Foxconn, another supplier of Apple’s iPhones, was similarly criticized in 2010 for extreme working conditions, following the suicide deaths of 14 factory workers.