SAN FRANCISCO — Apple Inc Chief Executive Tim Cook again stressed Tuesday that the iPhone and iPad maker takes labor conditions in its globe-spanning supply chain seriously, addressing persistent criticism that its production partners may be mistreating workers.

Cook, who took over from Silicon Valley icon Steve Jobs after his October death, said his company had a very "granular-level" understanding of its partners' treatment of workers.

"If we find a supplier that intentionally hires underage labor, its a firing offense," he told the audience at the Goldman Sachs technology conference in San Francisco.

Apple said on Monday the U.S. non-profit labor group Fair Labor Association has begun an "unprecedented" inspection of working conditions at its main contract manufacturers, including Foxconn's plants in southern China, as the maker of the iPhone continues to grapple with persistent image problems there.