WASHINGTON - The Federal Aviation Administration (FAA) said on Tuesday it was ordering immediate inspections of Boeing 777-200 planes with Pratt & Whitney PW4000 engines before further flights after an engine failed on a United flight on Saturday.

Operators must conduct a thermal acoustic image inspection of the large titanium fan blades located at the front of each engine, the FAA said.

The National Transportation Safety Board said on Monday that a cracked fan blade from the United Flight 328 engine that caught fire was consistent with metal fatigue.

"Based on the initial results as we receive them, as well as other data gained from the ongoing investigation, the FAA may revise this directive to set a new interval for this inspection or subsequent ones," the FAA said.

The damaged starboard engine of United Airlines flight 328 is seen following a Feb. 20 engine failure incident, Feb. 23, 2021.

In March 2019, after a 2018 United engine failure attributed to fan blade fatigue, the FAA ordered inspections every 6,500 cycles. A cycle is one take-off and landing.

South Korea's transport ministry said on Tuesday it had told its airlines to inspect the fan blades every 1,000 cycles following guidance from Pratt after the latest United incident.

An airline would typically accumulate 1,000 cycles about every 10 months on a 777, according to an industry source familiar with the matter.

The engine that failed on the 26-year-old Boeing 777 and shed parts over a Denver suburb on Saturday was a PW4000. The engines are used on 128 planes, or less than 10% of the global fleet of more than 1,600 delivered 777 widebody jets.

Boeing had recommended that airlines suspend the use of the planes while the FAA identified an appropriate inspection protocol, and Japan imposed a temporary suspension on flights after that incident.

United, the only U.S. operator, had temporarily grounded its fleet before the FAA announcement.