Greece's government appeared to be softening its stance toward its creditors, but European officials ruled out any deal before a Greek referendum this weekend.
Chubb Corp. soared 32 percent after the insurance company agreed to be acquired by Ace for about $28 billion.
The Dow Jones industrial average gained 160 points, or 0.6 percent, to 17,782 as of 9:35 a.m. Eastern time Wednesday.
The Standard & Poor's 500 index gained 16 points, or 0.8 percent, to 2,079. The Nasdaq composite climbed 45 points, or 0.9 percent, to 5,033.