(Adds Ryanair, Westjet comments)
* France's BEA sending experts to aid U.S. investigation
* Investigation expected to focus on metal fatigue
* FAA says engine inspection to take two hours
* Korean Air says 20-30 pct of 737 fleet of affected type
* More than 8,000 CFM56-7B engines on 737s worldwide
By Jamie Freed and Tim Hepher
SINGAPORE/PARIS, April 18 (Reuters) - Global airlines began inspecting some Boeing 737 engines on Wednesday as an investigation gathered pace into an explosion which killed a passenger on a Southwest Airlines flight in the first fatal U.S. airline accident in almost a decade.
European regulators earlier this month ordered checks following lengthy analysis of a similar non-fatal incident at Southwest two years ago but investigators warn it is too early to say whether the two problems are linked.
Southwest Flight 1380 made an emergency landing in Philadelphia on Tuesday after an engine ripped apart mid-air, shattering a window on the 737 and nearly sucking out a passenger. One of 144 passengers died.
French accident investigators said on Wednesday they were sending a team to assist the investigation led by the Washington-based National Transportation Safety Board because the engine was developed by a French-U.S. joint venture, CFM International.
France's Safran, which co-produces the engines along with General Electric, will also provide technical support, a spokesman for the BEA air accident agency added.
Safran shares reversed earlier slight losses to close up 0.5 percent in line with the market.
All recent Boeing 737s are powered by engines from CFM, a workhorse of the global airlines that has logged more than 350 million hours of safe travel but some of which were also being examined after the 2016 accident.
CFM says there are more than 8,000 of its CFM56-7B engines in operation on Boeing 737 passenger jets.
Although no cause has been ruled out, the first fatal U.S. airline accident since 2009 is expected to focus attention on the role of metal fatigue in engine accidents, which are rare.
An early review of Tuesday's failed Southwest engine found preliminary evidence of metal fatigue where a fan blade had broken off, Robert Sumwalt, chairman of the U.S. National Transportation Safety Board (NTSB), told reporters on Tuesday.
In August 2016, a Southwest flight made a safe emergency landing in Pensacola, Florida, after a fan blade separated from the same type of engine, and debris ripped a foot-long hole above the left wing. There too, investigators cited signs of metal fatigue.
"We are very concerned," the NTSB's Sumwalt said, referring to the overall challenge of detecting slow-developing metal fatigue.
"There needs to be proper inspection mechanisms in place to check for this before there's a catastrophic event."
Completing the latest investigation will take about 12-15 months, he said.
Southwest said it was speeding up inspections of all related engines, which it expected to complete within 30 days.
A person with knowledge of the situation said the airline was not inspecting all of its CFM56 engines, instead focusing on older ones - some 400 to 600 in total.
Investigators said on Tuesday they would be examining maintenance records of the airline, which operates one of the world's largest 737 fleets and has a strong safety record.
The 2016 incident prompted the U.S. Federal Aviation Administration (FAA) to propose ultrasonic inspections of similar fan blades and their replacement if needed.
Sumwalt said the NTSB would review whether the engines involved in Tuesday's incident might have been subject to that directive, which is not yet finalised.
European regulators, meanwhile, this month began requiring an inspection by early next year following a bulletin to airlines issued by CFM itself after the 2016 incident.
A person familiar with the matter said U.S. regulators were close to finalising a similar rule.
Korean Air Lines Co Ltd said it planned to carry out voluntary inspections of engines used on its entire 737 fleet by November.
About 20-30 percent of its 35 Boeing 737 jets use the same type of fan blade as the one on the Southwest jet.
Japan Airlines said two 737 jets in its fleet had engines with affected fan blades and inspections were due to be completed on Wednesday.
Ireland's Ryanair, Europe's largest 737 operator, said fewer than 70 of its 440 planes were fitted with identical CFM56-7B engines and that all had been inspected.
In Canada, WestJet Airlines said it planned to accelerate inspections of certain fan blades, while in Dubai, budget carrier flydubai said it had implemented the European directive ahead of the deadline.
The U.S. FAA has estimated that checks would require two hours per inspection.
Not all airlines operating 737s are affected.
Reporting by Jamie Freed in Singapore and Tim Hepher in Paris; Additional reporting by Jeffrey Dastin, David Shepardson, Joyce Lee, Sam Nussey, Byron Kaye, Alwyn Scott and Victoria Bryan; editing by Jason Neely and Adrian Croft