About 30 minutes into trading, the Dow Jones Industrial Average was up 0.7 percent to 24,481.32.

The broad-based S&P 500 gained 0.4 percent to 2,681.11, while the tech-rich Nasdaq Composite Index rose 0.6 percent to 7,159.84.

Investors were cheered by several major deals across the telecom, petroleum, travel and retail sectors.

The most prominent transaction was the T-Mobile US merger with Sprint, which established a giant with combined 131 million subscribers, virtually matching second-ranked AT&T and posing stiff competition to market leader Verizon Communications

Shares of Sprint sank 12.1 percent to $5.72, while T-Mobile rose 5.3 percent.

Another large deal was the $23.3 billion purchase of independent refiner Andeavor by Marathon Petroleum, forming the largest US refiner. Andeavor rose 14.2 percent, while Marathon fell 5.4 percent.

Meanwhile, Walmart rose 1.8 percent after agreeing to combine its British supermarket chain Asda with Saintsbury to create a £13-billion ($18-billion, 15-billion-euro) retail giant.

Dow member McDonald's surged 4.6 percent after reporting a 13.2 percent rise in first-quarter earnings to $1.4 billion in results that bested analyst expectations.

The good news overshadowed the uptick in the Federal Reserve's preferred inflation measure which hit the two percent target for the first time in just over a year, a prospect that in recent weeks has sent share prices lower.

Events later in the week include a Federal Reserve policy announcement and the April US jobs report.