Boeing Co said on Tuesday it plans to add extra staff and hire "a few hundred" temporary employees at an airport in Washington state where it is storing many grounded 737 MAX jetliners, a key step in its best-case plan for resuming deliveries to airline customers in October.

The world's largest planemaker, burning cash as one of the worst crises in its history stretches into a sixth month, said the workers will assist with aircraft maintenance and customer delivery preparations at Grant County International Airport.

The hiring plans are the first publicly detailed steps Boeing will take as it works to deliver hundreds of grounded 737 MAX jets to airlines globally, an undertaking that would amount to one of the biggest logistical operations in modern civil aviation.

Chicago-based Boeing has been unable to deliver any 737 MAX aircraft since the single-aisle plane was grounded worldwide in March after two fatal crashes in Indonesia and Ethiopia killed 346 people, cutting off a key source of cash and hitting margins.

Global airlines have had to cancel thousands of flights and use spare aircraft to cover routes that were previously flown with the fuel-efficient MAX, eating into their profitability. Many carriers have taken the MAX off their schedules late into the fall or early 2020.

Boeing reiterated on Tuesday that it was working toward getting the 737 MAX flying again commercially in the "early fourth quarter" after it wins approval of reprogrammed software for the stall-prevention system at the center of both crashes.

In late July, U.S. Federal Aviation Administration Deputy Administrator Dan Elwell declined to be pinned down on Boeing's previously stated target of October for entry into service.

"We don't have a timeline," Elwell said. "We have one criteria. When the 737 MAX has been - when the complications to it have been satisfactorily assessed, and the MAX is safe to return to service, that's the only criteria."

Boeing said it plans to move all the aircraft from Moses Lake, an eastern Washington location where it runs test flights, to facilities in the Seattle and Everett areas where its factories are located.

Hundreds of Boeing 737 MAX jets remain grounded worldwide, and Boeing has continued building the jets at a rate of 42 per month in the Seattle area. The U.S. planemaker is also storing freshly built aircraft outside its factories in Renton and Everett, around Seattle. It also has jets parked at a facility in San Antonio, Texas.

The total cost so far of the 737 MAX crisis is more than US$8 billion, mainly due to compensation the planemaker will have to pay airlines for the delayed deliveries and lower production.