NEW YORK (AFP) - Global retail giant Walmart, which has faced criticism over low wages and skimpy benefits for years, announced Thursday it would raise wages for 500,000 workers in the US.

The largest single employer in the United States said salaries for about 40 per cent of its US staff would be lifted to at least US$9 (S$12) an hour in April, US$1.75 above the federal minimum wage.

By Feb 1, 2016, US staff will be paid at least US$10 an hour.

The company also unveiled a training and education programme that will allow employees to earn a high school degree at no cost as well as pursue higher education at more affordable prices.

"These changes will give our US associates the opportunity to earn higher pay and advance in their careers," said Doug McMillon, chief executive of Wal-Mart Stores.

"We're pursuing a comprehensive approach that is sustainable over the long term."

The moves mounting criticism by labour unions and other groups that the company's low wages have pressured some staff to seek public assistance to make ends meet.

President Barack Obama has also made the issue of low pay and a growing income gap across the country a key policy focus.

Walmart said the increased spending on wages and education would weigh on future profits. The new programme will shave 20 cents per share from earnings in fiscal 2016.

Walmart projected fiscal 2015 earnings of US$4.70-US$5.05 a share compared with the US$5.19 expected by analysts.

In opening trade Walmart shares fell 2.7 per cent to US$84.00.