Apple Inc stepped up audits of working conditions at major suppliers last year, discovering multiple cases of underage workers, discrimination and wage problems.The iPhone and iPad maker, which relies heavily on Asian-based partners like China’s Foxconn Technology Group to assemble the vast majority of its iPhones and iPads, said it conducted 393 audits, up 72 per cent from 2011, reviewing sites where over 1.5 million workers make its gadgets.Apple in recent years has faced accusations of building its profits on the backs of poorly-treated and severely underpaid workers in China.That criticism came to the fore around 2010, after reports of suicides at Foxconn drew attention to the long hours that migrant labourers frequently endure, often for a pittance in wages and in severely cramped living conditions.Foxconn is the trading name of Hon Hai Precision Industry and employs 1.2 million workers across China.Under chief executive Tim Cook, who took over from Steve Jobs in 2011, Apple has taken new steps to improve its record and boost transparency, including the extensive audits of its sprawling supply chain. Last year, it agreed to separate audits by the independent Fair Labour Association.