Ratings agency Moody's Investors Service has upgraded the senior secured rating of Fortescue Metals Group Ltd subsidiary FMG Resources to B1 from B2, recognising its improving balance sheet. Moody's said the rating was assigned to the $US2 billion ($A2.07 billion) of secured notes issued by FMG Resources. Moody's has also assigned a B1 corporate family rating to parent entity Fortescue Metals Group. "The rating upgrade to B1 recognises the strengthening in Fortescue's financial profile due to the continued progress in its production ramp-up and the expectation for strong cash flow generation under its current annualised run rate," Moody's analyst Matt Moore said. Fortescue reached record levels of production in the June 2010 quarter, hitting a run rate of around 44 million tonnes per annum (Mtpa).