Uneasiness about of one of Boeing’s most popular jets mounted on Tuesday after the deadly crashes of two 737 Max 8 aircraft in under five months.

European authorities banned the planes, one of the most important aviation regulators in the world to change its stance over the plane’s safety. It followed earlier moves by aviation regulators in China, Indonesia, Singapore and Australia, as well as carriers in Africa, South America, and North America, to ground the jets.

With the decision by European authorities, roughly two-thirds of the 737 Max 8 aircraft in the world have been pulled from use since an Ethiopian Airlines plane crashed on Sunday, killing 157 people.

One country holding back: the United States.

Such groundings are rare in the United States. The Federal Aviation Administration, the American regulator, is typically hesitant to ground an entire fleet without concrete findings of an inherent design or manufacturing problem.

Early Tuesday, Dennis A. Muilenburg, the chief executive of Boeing, spoke to President Trump on the phone and made the case that the 737 Max planes should not be grounded in the United States, according to two people briefed on the conversation.

The investigation of the Ethiopia crash is in its initial phases, as the authorities analyze the plane’s flight-data and cockpit-voice recorders.

Which Airlines Fly the Boeing 737 Max 8

The jets typically make more than 8,500 flights per week worldwide.

The F.A.A. said on Monday that it would examine the data from the Ethiopia crash and act as necessary. But the agency added that it was too early to make a determination about what caused the fatal accident and cautioned against making comparisons to an October crash in Indonesia involving a Lion Air flight.

Boeing echoed the F.A.A.’s warning, with the company saying it was working closely with the American and Ethiopian authorities to investigate the cause of the crash. Boeing said it was committed to ensuring the safety and quality of its planes.

Southwest Airlines and American Airlines, the two carriers in the United States that use the 737 Max 8, also said they remained confident in the safety of the planes and planned to continue flying them. Both airlines have said they have analyzed data from their thousands of flights with the jets and found no reason to ground them.

[Read our ongoing coverage of developments related to Sunday’s crash.]

One overarching concern among regulators around the world was whether the system suspected of playing a role in the Lion Air crash had contributed to the latest accident.

Indonesian and American authorities have raised the possibility that a new system in the 737 Max — and pilots’ lack of familiarity with it — could have contributed to the Lion Air Flight 610 crash. The plane’s so-called maneuvering characteristics augmentation system, known as MCAS, was a new version that could automatically change the aircraft’s trajectory.

The similarities, with both planes crashing minutes after erratic takeoffs, are driving authorities to order the groundings. Despite Boeing and American authorities standing behind the plane, regulators and carriers elsewhere are opting to ban the plane in the absence of clear information.

Experience is one factor behind regulators’ decisions.

Regulatory standards for pilot experience vary widely from country to country. They also differ considerably from airline to airline within countries.

Pilots in the rapidly expanding aviation markets of East Asia and in developing countries tend to be much less experienced than their counterparts in the West. Li Jian, the deputy director of China’s Civil Aviation Administration, said the agency — the first to ground the 737 Max after the accident Sunday — worried about the challenges that could face pilots if an aircraft had unexpected difficulties.

The biggest worry involves possibly inaccurate signals from key flight instruments, Mr. Li said on Monday. Many pilots with less experience depend heavily on automatic systems to help them fly planes, and such systems in turn need reliable data.

“We are facing uncertainties about whether pilots have the courage or the capability to fly” if an aircraft has difficulties, Mr. Li said.

“When a pilot is operating manually, if he receives inaccurate signals, which has happened multiple times, it will bring trouble,” Mr. Li said. “As a government supervision department, we should make sure all problem are solved before we allow aircraft to be used.”

Mr. Li did not elaborate on when or where the inaccurate signals might have occurred on multiple occasions. The aviation regulator did not respond on Tuesday to a faxed request for comment.

Some carriers are bowing to pressure from passengers.

Comair, a South African airline company, initially said it would continue to fly the plane. But, in the face of travelers’ concerns, it said Monday that it was “removing the 737 Max 8 from its flight schedule.”

“The safety and confidence of our customers and crew is always our priority,” Wrenelle Stander, executive director of Comair’s airline division, said in a statement.

In the United States, calls to ban the plane are mounting.

Several senators, including Mitt Romney, Republican of Utah, and the Democrats Dianne Feinstein of California, Richard Blumenthal of Connecticut and Elizabeth Warren of Massachusetts, have called on the F.A.A. to ground the Boeing planes until the investigation into the Ethiopia crash is completed.

“The world has now witnessed the second tragic crash of one of these planes in less than six months. While we do not know the causes of these crashes, serious questions have been raised about whether these planes were pressed into service without additional pilot training in order to save money,” Ms. Warren, who is running for president, said in a statement. “Today, immediately, the F.A.A. needs to get these planes out of the sky.”

Follow Keith Bradsher on Twitter: @KeithBradsher.

David Gelles, Christine Negroni, Amie Tsang contributed reporting. Ailin Tang contributed research.

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