Investors have been buoyed by signs the Chinese government will contain any fallout from Evergrande's debt crisis after fears about a full-on implosion throttled the market on Monday, said LBBW's Karl Haeling.

"This is just increased optimism that China will not allow Evergrande to just completely collapse in an uncontrolled liquidation of assets," Haeling said.

Markets are also pleased that Federal Reserve Chair Jerome Powell maintained a broadly dovish tone at Wednesday's press conference, even as he signaled the central bank was near to starting the tapering of asset purchases.

Those benign factors helped stocks shrug off somewhat disappointing US labor data showing an uptick in weekly unemployment claims.

About 15 minutes into trading, the Dow Jones Industrial Average was up 1.2 percent to 34,673.83.

The broad-based S&P 500 gained 1.0 percent to 4,438.80, while the tech-rich Nasdaq Composite Index climbed 0.7 percent to 14,998.76.

Among individual companies, Salesforce.com shot up 4.5 percent as it raised its revenue forecast for the 2022 and 2023 fiscal years.