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By Corky Siemaszko

The Federal Aviation Administration on Monday said U.S. airlines can still fly the Boeing jet model that has been involved in two crashes in less than six months.

The FAA affirmed the airworthiness of the Boeing 737 Max 8 and the 737 Max 9 aircraft as the agency faces concerns about the safety of the jets following an Ethiopian Airlines crash that killed all 157 people aboard Sunday.

The 737 Max 8 that crashed in Ethiopia is that same model that went down in Indonesia in late October, killing 189.

The FAA also said Monday that it will closely examine all crash data in Ethiopia and "will take appropriate action" if the data indicates the need to order the jets grounded.

"All data will be closely examined during this investigation, and the FAA will take appropriate action if necessary," the agency said in a statement.

The agency also noted that reports are drawing similarities between this accident and the Indonesia crash, but cautioned that "this investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions."

Besides the FAA, U.S. airlines and Boeing Co. were all coming under pressure Monday to keep 737 Max jets out of the air while the crash investigation is ongoing.

Sen. Dianne Feinstein, D-Calif., was among those calling for the FAA to ground them.

“This aircraft model represents only a small fraction of the domestic fleet, and several other countries have already taken this important step, including China and Indonesia," she said.

The 737 Max 8 and the newer Max 9 are “the fastest-selling airplane in Boeing history,” the company says on its website. It says it has received “nearly 4,700 orders from more than 100 customers worldwide” for the jets.

The company has built 329 of the 737 Max 8 series and 21 of the Max 9 models, according to the FAA.

But Jim Hall, a former head of the National Transportation Safety Board, said on MSNBC that Boeing should ground all of these aircraft until they can be checked for safety.

Hall noted that Sunday’s catastrophe in the Ethiopian capital of Addis Ababa happened less than six months after the Lion Air crash in Jakarta, Indonesia, in late October.

“My personal feeling on this is that Boeing aircraft company ought to voluntarily itself ground these aircraft because of the similarities between these two accidents,” Hall said. “It’s a brand new aircraft, and we had such a remarkable safety record in aviation, that I think this blip needs to be addressed by the manufacturer, and they need to do a detailed — a detailed look, and with these black boxes available, that’ll be done in a short term.”

The union representing flight attendants meanwhile said Boeing, U.S. airlines, the NTSB and the FAA need to address safety concerns.

"Crew and passengers are expressing concern about the 737 MAX 8 following a second crash, with similar characteristics to the Lion Air Flight 610 crash," Association of Flight Attendants-CWA International President Sara Nelson said in a statement. "It is vitally important that U.S. airlines work with Boeing, the FAA, and the NTSB to address concerns and take steps to ensure confidence for the traveling public and working crews."

In a statement, Boeing said it was sending a team to Ethiopia to provide assistance, but it had not as yet called for grounding the new planes.

“We extend our heartfelt sympathies to the families and loved ones of the passengers and crew on board and stand ready to support the Ethiopian Airlines team,” the company said in its latest statement. “A Boeing technical team will be traveling to the crash site to provide technical assistance under the direction of the Ethiopia Accident Investigation Bureau and U.S. National Transportation Safety Board."

U.S. airlines that fly 737 Max 8 planes also have not indicated any immediate plans to stop flying the jets.

In response to Twitter messages from apparently worried passengers, Southwest Airlines confirmed it has 34 of the 737 Max 8 version of the planes and said the company is “confident in the safety of our fleet.”

"As Southwest operates a fleet of 34 Boeing 737 MAX 8 aircraft, we have been in contact with Boeing and will continue to stay close to the investigation as it progresses," the airline said in a statement. "We remain confident in the safety and airworthiness of our entire fleet of more than 750 Boeing 737 aircraft, and we don’t have any changes planned to 737 MAX operations."

American Airlines said it has 24 of the 737 Max 8 aircraft and that it will “closely monitor the investigation in Ethiopia.”

“We have full confidence in the aircraft and our crew members, who are the best and most experienced in the industry,” American said in a statement.

In a subsequent statement, the airline said it continues to have "confidence in the safe operation of all of our aircraft, including the 737 MAX 8."

United Airlines said it doesn’t have any 737 Max 8 jets but has 14 of the 737 Max 9 versions in its fleet.

“We have made clear that the Max aircraft is safe and that our pilots are property trained to fly the aircraft safely,” a United Airlines spokesman said.

The Aviation Capital Group, which leases aircraft to airlines around the world, is also a Boeing customer, according to the company website. It was not immediately clear whether it owns the 737 Max 8, the Max 9 or both. NBC News reached out to Aviation Capital for comment but did not immediately hear back.

Canadian airlines such as WestJet, Sunwing Airlines and Air Canada also have 737 Max 8 jets or the Max 9 version in their fleets as does the Mexican airline Aeromexico, according to the Boeing website.

The Air Canada pilots' association called on the government "to take proactive action to ensure the safety of the Canadian traveling public."

"No-one is more invested in the well being of passengers and crew that those who operate the aircraft upon which they fly," the pilots' association said in a statement.

China and Indonesia have joined Ethiopia in halting the use of 737 Max 8 and Max 9 jets while the crash investigation is underway.

Chinese aviation authorities suspended the operation of all 737 Max 8 planes by domestic airlines such as Xiamen and Shandong.

In addition, Cayman Airways, which operates in the Caribbean and has regular flights to and from the U.S., said in a statement it too was suspending all 737 Max 8 flights “until more information is received.”