The Federal Aviation Administration stood by the safety of the Boeing 737 MAX on Tuesday, saying it hasn't found any issues at fault with the jetliner that would merit a grounding order, even as Britain, France and Germany joined the list of countries banning it from their skies.

"Thus far, our review shows no systemic performance issues and provides no basis to order grounding the aircraft," said Acting FAA Administrator Daniel K. Elwell in a statement.

He said the FAA continues to investigate the aircraft in the wake of Sunday's crash of an Ethiopian Airlines 737 MAX 8 and that the agency will "take immediate and appropriate action" if something crops up.

More than two dozen airlines around the world have grounded the planes since the crash of Flight 302 that killed all 157 aboard, the second deadly crash of a MAX 8 in less than five months.

The Boeing 737 MAX fleet includes 74 flown by domestic carriers, among almost 400 worldwide.

Boeing stressed its “full confidence in the safety" of the planes Tuesday.

“The Federal Aviation Administration is not mandating any further action at this time, and based on the information currently available, we do not have any basis to issue new guidance to operators,” Boeing said in a statement.

The MAX 8 was 4 months old and minutes into a Nairobi-bound flight from Addis Ababa on Sunday when it nosedived into a field. In October, a Lion Air plane of the same model crashed into the Java Sea – 12 minutes after departing from the airport in Jakarta, Indonesia. None of the 189 passengers and crew survived.

Earlier in the day before Elwell issued his statement, the FAA said "external reports are drawing similarities between this accident and the Lion Air Flight 610 accident," but it added "this investigation has just begun, and to date, we have not been provided data to draw any conclusions or take any actions."

The agency said it expects to require Boeing to complete MAX 8 flight control system enhancements – prompted by the Lion Air crash – by month's end.

The United Kingdom Civil Aviation Authority said it did not have sufficient information about the latest crash.

"We have, as a precautionary measure, issued instructions to stop any commercial passenger flights from any operator arriving, departing or overflying UK airspace," the authority said in a statement.

China, Indonesia, Malaysia, Singapore, Oman and Australia are among other nations that grounded the planes. Turkish Airlines, Polish carrier LOT and Norwegian Air Shuttle joined more than two dozen airlines parking their MAX 8s Tuesday.

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There has been pushback in the USA as well.

Transport Workers Union Local 556 represents flight attendants on Southwest Airlines, which flies 34 of the planes. TWU issued a statement saying the planes should be grounded pending further investigation. "People must always be put over profits," the union tweeted.

The Association of Flight Attendants, which says it has nearly 50,000 members and is part of the Communications Workers of America, called for grounding the planes.

Sen. Mitt Romney, R-Utah, urged the FAA to ground the planes "out of an abundance of caution for the flying public" until safety can be assured. Sens. Elizabeth Warren, D-Mass.; Dianne Feinstein, D-Calif.; and Richard Blumenthal, D-Conn., also called for the FAA to ground the MAX 8.

Sen. Ted Cruz, R-Texas, in joining the growing chorus, said he would hold a hearing into the causes of the crashes as chairman of the Senate Subcommittee on Aviation and Space.

The FAA said it was providing technical support to the Ethiopian Accident Investigation Bureau. Boeing said it was also aiding the investigation.

The plane was delivered to the airline in November, had flown 1,200 hours and had undergone a maintenance check Feb. 4. The pilot, who had more than 8,000 hours of flight experience, had issued a distress call and tried to return to the airport.

The "black box" voice and data recorders were found, raising hopes that investigators would soon learn more details of the crash. Airline CEO Tewolde GebreMariam told CNN the pilots told air traffic control they were having “flight control problems” before the crash.

More:3 questions don't have answers following Boeing MAX 8 crash

The stakes for Boeing are high: Airlines have ordered 4,661 more of the planes – the newest version of the 737 and best-selling airliner ever.

Southwest and American fly the plane, and both expressed confidence in their fleets. Southwest, which has 34 of the planes and is adding more, said on Twitter that the airline had flown 31,000 flights on 737 MAX planes and plans on "operating those aircraft going forward."

More:Boeing 737 Max: What you should know if you're booked on a flight

President Donald Trump weighed in on Twitter but took no position on grounding the planes. Trump said planes have become so complex that "computer scientists from MIT" are required to fly them.

"Split second decisions are needed, and the complexity creates danger," Trump added. "I don’t want Albert Einstein to be my pilot. I want great flying professionals that are allowed to easily and quickly take control of a plane!"

Trump spoke by phone with Boeing CEO Dennis Muilenburg, said two officials speaking on condition of anonymity because it was a private conversation. They declined to detail the conversation, except to say it came after Trump's tweet Tuesday.

Contributing: Bart Jansen, David Jackson, Donovan Slack and Tom Vanden Brook