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Qantas has been forced to delay flights between Australia and South Africa after the US government warned the airline about the risk of SpaceX rocket parts re-entering the atmosphere in the southern Indian Ocean.

Australia’s flag carrier said on Tuesday it had postponed several flights between Sydney and Johannesburg over the past few weeks, with delays lasting between one and six hours.

Ben Holland, the head of the operations center of Qantas, said that the timings of the recent re-entry into the atmosphere of parts of the rockets owned by Elon Musk’s SpaceX “shifted at late notice”, which forced the airline to delay some flights before their scheduled departure.

He added: “We are meeting with SpaceX to see if they can refine the locations and time windows for rocket re-entries to minimize future disruption to our passengers en route.”

Few airlines fly across the remote southern Indian Ocean, however Qantas has been operating the South Africa-to-Australia service – known as the Wallaby route – since the early 1950s.

It is a route frequently used by tourists, South Africans living in Australia and mining industry executives.

The Federal Aviation Administration, the US regulator, licenses all commercial space rocket launches and re-entry within the US, as well as those made by American companies abroad.

Rocket launches are often carefully calibrated to ensure that non-reusable parts scatter in remote areas of the ocean. The exact location will depend on the flight, with planes and ships told to avoid the re-entry zone.

Qantas’ warning comes as the launch rate is expected to increase with more new rockets entering the market.

Last year, rocket launch attempts hit a record 259 worldwide, with 256 successful.

SpaceX accounts for more than half of that number and is expected to increase the pace of launches this year as it brings its giant Starship rocket into service. This week, it is expected to make the seventh test flight even though the date has been pushed back.

Rival rocket company Blue Origin, backed by billionaire Jeff Bezos, is also scheduled to fly its New Glenn rocket in the coming days after canceling its first flight on Monday due to an icing issue.

There are times when parts fall back to the ground outside of controlled zones. Last year, SpaceX’s Falcon 9 was grounded after three incidents involving parts that re-entered outside the controlled splashdown zone.

Qantas shares fell 2 percent in Australia after it revealed flight delays.

SpaceX has been contacted for comment.