US Treasury Secretary Timothy Geithner said the US GDP figures released on Thursday were "encouraging," but that it would be up to the private sector to turn one quarter of positive growth into a sustainable recovery.

In a speech here Geithner said the economic figures reflected a broad-based recovery, though he warned it was still "early" in the process.

The quarterly results were "broad and strong," he said in remarks to the Economic Club of Chicago.

"It wasn't just cash for clunkers and it wasn't just the direct effects of stimulus. You saw consumption stronger, investment stronger, housing, construction stronger, exports rising quite rapidly."

In percentage terms, the US economy grew at a seasonally adjusted 3.5 percent annual rate in the July-September period from the previous quarter.

The growth exceeded analyst expectations and marked the strongest quarter since the third quarter of 2007 when a US subprime mortgage crisis triggered a global financial meltdown.

Geithner said the administration of President Barack Obama could not "borrow and spend" its way out of this recession, and was looking to the private sector, and specifically exports and private investment, to drive the economy going forward.

He said the US economy had "stabilized," that there had been a significant improvement in confidence in the financial system and most large businesses were able to access the credit they needed. But he conceded some sectors were still hurting.

"If you are a small business, if you are in parts of the economy affected by the crisis, it's still very difficult. You are affected by the classic risk of the credit crunch slowing the pace of recovery."