(Reuters) - Amazon.com is raising the pay of its fulfillment and transportation staff in the United States by at least $1.50 an hour and handing out free Prime memberships as benefits, it said on Wednesday.

The company will spend over $2.2 billion on the hikes that will take the base salaries of hourly workers to more than $22 an hour, and more than $29 an hour including benefits.

Amazon's decision comes as one of the biggest employers in the US faces pressure from warehouse staff for better contracts and improved working conditions.

In June, the Amazon Labor Union - the first group of company workers to organize at a US warehouse - voted to affiliate with the International Brotherhood of Teamsters, one of the largest US labor unions.

The hikes will roll out starting this month and could help Amazon hire and retain staff as it heads into the key holiday season, typically the busiest shopping period of the year.

The Prime membership will be added to the benefits package early next year.

Amazon employs more than 800,000 full-time and contract workers in fulfillment and transportation roles in the US, according to the company.

In 2018, it paid these workers a starting wage of $15 per hour.