President Donald Trump lashed out against Amazon for the sixth time in a week, during an Air Force One trip on Thursday back from an event in West Virginia.

"They are not on an even playing field," Trump told reporters, contending other retailers are at a disadvantage to the e-commerce giant. "The playing field has to be level for everybody," he said.

The president took a moment to criticize The Washington Post, which is owned by Amazon CEO Jeff Bezos. Trump repeated his false claim from earlier on Thursday that it acts as a "lobbying effort" for the e-commerce company.

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He then switched gears, mentioning how "the Post Office is not doing well with Amazon."

He noted that the Supreme Court is expected to decide later this year on whether all Amazon transactions will be subject to state sales tax.

"We'll see what happens," the president said.

Trump has been an extremely vocal critic of Amazon in recent weeks, claiming that the company is taking advantage of its deal with the U.S. Postal Service to hurt brick-and-mortar retailers.

Some observers have argued the partnership between USPS and Amazon has been positive for the delivery service.

Amazon shares edged down in extended trading.