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.
European markets also rose. Germany's DAX increased 2.5 percent and France's CAC 40 2.4 percent.