NEW YORK (BLOOMBERG) - US stocks fluctuated as investors weighed growth prospects for the world's biggest economy amid rising bets on continuing policy support from the Federal Reserve.

The S&P 500 Index rose less than 0.1 per cent to 2,174.04 at 9.33 am in New York (9.33 pm Singapore time), after capping a fifth monthly gain on Friday, the longest such streak since 2014.

"The market's in that uncertain zone as we try to see what earnings do for us," said Bruce McCain, who helps oversee US$35 billion (S$46.8 billion) as chief investment strategist at Key Private Bank in Cleveland. "The GDP report reminds people that things are not better yet. The question is whether or not they're willing to keep prices moving higher on the expectation that things will get better or whether they're going to insist on seeing better news as we move through this."

Traders cut the chances of a rate hike in September to 20 per cent from 26 per cent a week earlier, and pushed back the first month with at least even odds for an increase to June 2017 from March after a report Friday showed the US economy stumbled in the first half of 2016 as companies retrenched.

Still, Dallas Fed President Robert Kaplan said a rate move higher at the next policy meeting in September is still possible. The Fed last week held borrowing costs unchanged as forecast but reiterated its intention to raise them gradually.

Investors will continue to asses the strength of the US economy with reports this week offering measures on consumer spending, services industry growth, factory orders and the government's monthly payrolls report set for Friday. Data today is forecast to show manufacturing activity expanded in July, while construction spending rebounded in June.