NORTH HAVEN, Conn. (AP) — Robots haven't replaced warehouse workers yet, but they're here — and they need some human supervision.

Doing your job side-by-side with robots isn't easy. According to their makers, the machines should take on the most mundane and physically strenuous tasks. In reality, they're also creating new forms of stress and strain in the form of injuries and the unease of working in close quarters with mobile half-ton devices that direct themselves.

"They weigh a lot," Amazon worker Amanda Taillon said during the pre-Christmas rush at a company warehouse in Connecticut. Nearby, a fleet of 6-foot-tall roving robot shelves zipped around behind a chain-link fence.

Taillon's job is to enter a cage and tame Amazon's wheeled warehouse robots for long enough to pick up a fallen toy or relieve a traffic jam. She straps on a light-up utility belt that works like a superhero's force field, commanding the nearest robots to abruptly halt and the others to slow down or adjust their routes.

“When you’re out there, and you can hear them moving around, but you can’t see them, it’s like, ‘Where are they going to come from?’,” she said. “It’s a little nerve-racking at first.”

Taillon says she’s gotten used to working with robots — something Amazon and its rivals are increasingly requiring warehouse employees to do. Amazon now has more than 200,000 robotic vehicles it calls “drives” that are moving goods through its delivery-fulfillment centers around the U.S. That's double the number it had last year and up from 15,000 units in 2014.

Its rivals have taken notice. Many are adding their own robots in a race to speed up productivity and bring down costs.

Without these fast-moving pods, robotic arms and other forms of warehouse automation, retailers say they wouldn't be able to fulfill consumer demand for packages that can land on doorsteps the day after you order them online.

But while fears of fully automated warehouses haven't come to fruition, there are growing concerns that keeping up with the pace of the latest technology is taking a toll on human workers' health, safety and morale.

Warehouses powered by robotics and artificial intelligence software are leading to human burnout by adding more work and upping the pressure on workers to speed up their performance, said Beth Gutelius, who studies urban economic development at the University of Illinois at Chicago and has interviewed warehouse operators around the U.S.

Much of the boom in warehouse robotics has its roots in Amazon's $775 million purchase of Massachusetts startup Kiva Systems in 2012. The tech giant re-branded it as Amazon Robotics and transformed it into an in-house laboratory that for seven years has been designing and building Amazon's robot armada.