WASHINGTON (Reuters) - 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.

(Reporting by David Shepardson; Editing by Himani Sarkar)