Walmart CEO Doug McMillon told shareholders and presidential hopeful Bernie Sanders on Wednesday the federal minimum wage of $7.25 per hour is "too low."

"It's time for Congress to put a thoughtful plan in place to increase the minimum wage," McMillon said at the start of the giant retailer's annual meeting in Bentonville, Arkansas. "Any plan should take into account phasing and cost-of-living differences to avoid unintended consequences."

At the meeting, Sanders called for wage hikes for Walmart workers, accusing Walmart of paying "starvation wages."

The world's biggest retailer, which employs 1.5 million people in the U.S., has faced increasing pressure to raise its pay. Amazon CEO Jeff Bezos, in his annual letter to shareholders in April, issued a challenge to other retailers, not naming which ones specifically, to match its wages and benefits.

Walmart's minimum wage of $11 an hour, set in January 2018, is still below Amazon's, which was hiked to $15 in November. "Do it! Better yet, go to $16," Bezos said in the letter.

Walmart, however, has said its average, hourly compensation including benefits comes out to be more than $17.50.

Walmart has also been sweetening its perks to include benefits such as the opportunity for employees to pay only $1 a day to earn college degrees in fields related to business and technology.