SAN FRANCISCO (AFP) - Amazon began advertising its plan Tuesday to recruit on-demand delivery workers, in a move that challenges startups in the sizzling sector.

The US online giant launched its Flex webpage seeking people to deliver Amazon packages on a contract basis, saying it can produce earnings of US$18 to US$25 (S$25.70 to S$35.70) an hour.

The programme is now available in Amazon's hometown of Seattle, Washington, and "coming soon" to other cities including New York, Baltimore, Miami, Dallas, Austin, Chicago, Indianapolis, Atlanta, and Portland.

"Be your own boss: deliver when you want, as much as you want," the website says.

Amazon enters a field crowded with delivery startups like Postmates and Instacart, and similar services from the ride-sharing giant Uber.

Those who join will need to be at least 21 years old, with a driver's license, vehicle and Android smartphone, and will undergo a background check.

They will be able to choose blocks of two, four or eight hours to work.

Amazon's drive comes amid a growing trend of on-demand employment replacing traditional jobs. While offering more flexibility, the workers generally lack benefits such as unemployment or disability insurance.

The move comes with Amazon expanding its options for deliveries of groceries and other items in many locations, with same-day delivery for many items.