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The weakening of Irma, the second major natural disaster to hit the United States after Hurricane Harvey, allayed concerns about the severity of its financial impact.
"Investors have gained confidence that the worst is over and concerns over Irma's economic impact seem to have disappeared for the moment," said Andre Bakhos, managing director of Janlyn Capital in Bernardsville, New Jersey.
All eyes will be on Apple as it prepares to unveil the 10th anniversary edition of the iPhone, whose sales will also have repercussions on its rivals and many suppliers.
Apple shares dipped 0.3 percent ahead of the event, scheduled to start at 1:00 p.m. ET (1700 GMT).
At 9:39 a.m. ET, the Dow Jones Industrial Average was up 64.64 points, or 0.29 percent, at 22,122.01 and the S&P 500 was up 6.76 points, or 0.27 percent, at 2,494.87.
The Nasdaq Composite was up 16.20 points, or 0.25 percent, at 6,448.46.
Nine of the 11 major S&P sectors were higher, led by a 1.03 percent rise in materials index.
DowDupont rose 3 percent after the company said it was making changes to a plan of splitting itself into three. The stock gave the biggest boost to the S&P.
The market also seemed to shrug off North Korea's threat that the United States would soon face the "greatest pain" it had ever experienced in the wake of the U.N. stepping up sanctions against Pyongyang on Monday due to its nuclear tests.
North Korea on Tuesday rejected the U.N. Security Council resolution, which imposed a ban on the country's textile exports and capped crude oil imports.
"Although North Korea is a threat and can make the market nervous because of uncertainty, more and more global powers are coming on board, taking the pressure off of just the U.S. dealing with it. About that, the investors feel good," Bakhos said.
Sage Therapeutics slumped more than 19 percent after its drug to treat a life-threatening seizure disorder failed in a late-stage trial.
Teva rose 5.24 percent after the drugmaker agreed to sell its contraceptive brand, Paragard, for $1.1 billion to a unit of Cooper Cos, whose stock was flat.
In economic data, the Labor Department is expected to release its monthly job openings and labor turnover survey report for July at 10:00 a.m. ET. Job openings rose to 6.163 million in June.
Advancing issues outnumbered decliners on the NYSE by 1,626 to 844. On the Nasdaq, 1,396 issues rose and 867 fell.