Bad loans are weighing heavily on China's top commercial banks this year, and are likely to hit profitability and asset quality, a report released by PwC claimed on Thursday.

The study revealed that total overdue loans among the country's top 10 listed banks had increased to 486.5 billion yuan ($79.3 billion) by the end of last year, up 29 percent from 2011.

The average overdue loan ratio rose to 1.21 percent from 1.06 percent, "a considerable deterioration", said Jimmy Leung, PwC's banking and capital markets leader for China.