Stocks will continue to have an upward bias until the start of 2020, when investors will look for more specific details in the trade agreement, said Michael Arone, chief investment strategist at State Street Global Advisors in Boston.

"Investors are essentially waiting to see what happens next, moving from a 'tell-me-something-good' environment to a 'show me something good' environment," Arone said.

Further cementing optimism on the strength of the US economy, data on Thursday showed the number of Americans filing applications for unemployment benefits dropping from more than a two-year high last week.

The market also shrugged off US President Donald Trump's impeachment, as the Republican-controlled Senate is widely expected not to convict him and remove him from office.

At 11:50 a.m. ET the Dow Jones Industrial Average was up 109.38 points, or 0.39 per cent, at 28,348.66, the S&P 500 was up 9.79 points, or 0.31 per cent, at 3,200.93 and the Nasdaq Composite was up 41.44 points, or 0.47 per cent, at 8,869.18.

Conagra Brands jumped 18.1 per cent, on track for its best day in over 30 years, after the Slim Jim maker beat quarterly sales and profit estimates.

Conagra's results also lifted other packaged food makers and helped boost the consumer staples sector 0.6 per cent.

Micron Technology Inc gained 3.5 per cent after signaling a recovery in its business in 2020 and saying it had received licenses to supply some products to Huawei.

Advancing issues outnumbered decliners by a 1.43-to-1 ratio on the NYSE and by a 1.31-to-1 ratio on the Nasdaq.