The move follows criticism that Amazon has faced about reportedly harsh conditions for workers at its fulfillment centers, where orders received online are packaged and shipped out to customers.

"Many of our fulfillment center employees will choose to build their careers at Amazon. For others, a job at Amazon might be a step towards a career in another field. We want to make it easier for employees to make that choice and pursue their aspirations," CEO Jeff Bezos wrote in the letter.

Under the training benefit program, called , Amazon will pay up to US$2,000 a year for four years for employees to undertake vocational training for high-demand jobs that pay relatively well.

The company will pay for training in fields such as aircraft mechanics, computer-aided design, machine tool technology, medical laboratory science and nursing, it said. Amazon drew from data from the Bureau of Labor Statistics to help it determine which careers to support.

Amazon's fulfillment centers drew criticism after an investigative last September depicted workers logging long hours in sweltering heat at one location. Protestors raised the issue again at the company's annual shareholders meeting in May, and CEO Jeff Bezos promised to install air conditioning in the warehouses.