The deficit equalled about 7 percent of US economic output, down from 8.7 percent in 2011, 9 percent in 2010 and 10.1 percent in 2009, but it was still greater than in any other year since 1947, the non-partisan Congressional Budget Office said.

Economists generally consider any deficit that exceeds 3 percent of US gross domestic product to be unsustainable in the long term.

CBO said a $75 billion surplus September surplus helped to hold the full-year fiscal 2012 deficit to $1.09 trillion, compared with a $1.297 trillion deficit in fiscal 2012.

The September surplus was just the second month in the black for the US government since September 2008, when the country was in the throes of a financial crisis. The September data was buoyed by strong quarterly corporate income tax payments and $7 billion from the sale of shares in bailed-out insurer American International Group.

The US Treasury is expected to release official final figures for the year ended Sept. 30 next week.