Amazon founder and CEO Jeff Bezos. David Ryder/Getty Images Amazon significantly expanded its army of warehouse robots in 2016, according to a report by The Seattle Times.

The newspaper — based in the same city as Amazon's global headquarters — wrote last week that the e-commerce giant now has 45,000 robots across 20 fulfillment centres.

That would be a 50% increase from the same time the year before, when the company said it had 30,000 robots working alongside 230,000 people.

Amazon bought a robotics company called Kiva Systems in 2012 for $775 million (£632 million). Kiva's robots automate the picking and packing process at large warehouses in a way that stands to help Amazon become more efficient. The robots — 16 inches tall and almost 145 kilograms — can run at 5 mph and haul packages weighing up to 317 kilograms.

When Amazon acquired Kiva, Phil Hardin, Amazon's director of investor relations, said: "It's a bit of an investment that has implications for a lot of elements of our cost structure, but we're happy with Kiva. It has been a great innovation for us, and we think it makes the warehouse jobs better, and we think it makes our warehouses more productive."

Amazon also uses other types of robots in its warehouses, including large robotic arms that can move large pallets of Amazon inventory.

The company has been adding about 15,000 robots year-on-year, based on multiple reports. At the end of 2014, Amazon said it had 15,000 robots operating across 10 warehouses. In 2015, that number rose to 30,000, and now Amazon has 45,000.

Kiva robots moving inventory at an Amazon fulfillment center in Tracy, California. Reuters

"We've changed, again, the automation, the size, the scale many times, and we continue to learn and grow there," Amazon CFO Brian Olsavsky said at a conference last April, according to The Times.

Olsavsky added that the number of robots used varies from warehouse to warehouse, saying that some are "fully outfitted" in robots, while others don't have "robot volume" for economic reasons.

Beyond the warehouse, Amazon is also looking at automating other aspects of its business. In December, the company announced it had made its first delivery by an automated drone in the UK. It's also filed a patent that would allow it to use automated drones to deliver packages from large airships in the future.