Review said officials oversaw design changes "in a way that failed to achieve the full safety benefit."

US aviation regulators were slammed Friday for allowing design changes to Boeing's 737 MAX aircraft that have been implicated in two crashes causing the deaths of more than 300 people.

The Federal Aviation Administration failed to stick to its own rules, followed out-of-date procedures and lacked the manpower and expertise to properly oversee the alterations, a panel of worldwide experts found.

The Joint Authorities Technical Review (JATR) was put together in March after a 737 MAX run by Ethipoian Airlines crashed killing all 157 onboard.

That came after the same make of jet flown by Lion Air plunged into the ocean off Indonesia in October, with the loss of 189 people, in similar circumstances.

The tragedies led to the airliner being grounded and a step-up in oversight of Boeing from the FAA and international regulators.

But the FAA came in for harsh criticism from the JATR, which said there was "an inadequate number of FAA specialists" in place to oversee a new design of the 737 MAX and they "had inadequate awareness" of the system implicated in the crashes.

It said officials oversaw design changes "in a way that failed to achieve the full safety benefit."

Their damning 69-page report also found that Boeing had put pressure on some of its staff who had FAA authority to pass the updated designs.

The JATR panel included members of the FAA as well as NASA and other regulators from around the world.

In response to the report, FAA Administrator Steven Dickson said: "Today's unprecedented US safety record was built on the willingness of aviation professionals to embrace hard lessons and to seek continuous improvement.

"We welcome this scrutiny and are confident that our openness to these efforts will further bolster aviation safety worldwide. The accidents in Indonesia and Ethiopia are a somber reminder that the FAA and our international regulatory partners must strive to constantly strengthen aviation safety."