U.S. airlines, including American and United on Wednesday asked the federal government not to use their flights to transport children who were separated from their families under a controversial immigration policy.

The airlines responded after a public outcry over the separations of children from their parents and reports that claimed some of the children were transported on U.S. flights. More than 2,300 migrant children were separated from their parents at the U.S-Mexico border from May 5 and June 9, the Department of Homeland Security said Tuesday.

President Donald Trump on Wednesday signed an executive order that he said would keep migrant families together if the adult members are detained after trying to enter the U.S. illegally. But the policy that Trump has appeared to backed away from put the spotlight on U.S. airlines, which are regularly contracted by the U.S. government.

The Houston Chronicle published on Tuesday an adapted Facebook post by Hunt Palmquist, whose bio said he is a Dallas-based flight attendant for a major U.S. carrier. He said that "little children whose faces were full of fear, confusion, sadness and exhaustion left me somewhat traumatized as it occurred to me a few weeks later that I might as well have been a collaborator in their transport.

"I will immediately remove myself from the trip due to the nature of this unconscionable act by my government and my employer's complicity," he wrote.

Another Facebook post that said it was passed along from a flight attendant said more than a dozen migrant children appeared to have been recently transported on an overnight flight from Phoenix to Miami.

A spokesman for the Department of Homeland Security, Tyler Houlton, in a tweet called it "unfortunate" that the airlines "no longer want to partner with the brave men and women of DHS to protect the traveling public, combat human trafficking, and to swiftly reunite unaccompanied illegal immigrant children with their families."

Buckling to a false media narrative only exacerbates the problems at our border and puts more children at risk from traffickers. We wish the airlines would instead choose to be part of the solution.

American Airlines said it had "no knowledge" the federal government used the airline to transport migrant children who were separated from their parents under the immigration policy but asked the government to "immediately refrain" from using the airline for that purpose.

"We have no desire to be associated with separating families, or worse, to profit from it," American said in a statement. "We have every expectation the government will comply with our request and we thank them for doing so."

The carrier said it has carried refugees for nonprofits and the government.

United CEO Oscar Munoz said the airline has not seen any evidence that these children separated from their parents were flown on United's planes, but the airline said it told the government it "should not" transport migrant children who were separated from their parents on its flights.

"Our company's shared purpose is to connect people and unite the world," Munoz said in a statement. "This policy and its impact on thousands of children is in deep conflict with that mission and we want no part of it."

The immigration policy has shaken some airline crew members, as well.

"This national discussion and response is being felt on the planes and discussed among crews. Some are struggling with the question of participating in a process that they feel deeply is immoral," said Sara Nelson, president of the Association of Flight Attendants, the labor union that represents flight attendants at United Airlines, Alaska Airlines and others.

"Airlines are often a first point of contact for any public debate on these issues because all of society passes through our aircraft," Nelson said.

Privately-held, low-cost carrier Frontier Airlines also asked the U.S. government not to use its planes to fly children separated from their parents under the policy and that it is not aware if its planes have been used for that purpose.

Southwest Airlines said it "does not have evidence" that it sold tickets that were used to transport detained children and said it doesn't want to be involved in that business. Spirit Airlines said it will not "knowingly participate in transporting migrant children away from their parents.

Delta Air Lines said that the policy of separating migrant families "do not align with Delta's core values" and it applauded the Trump for signing the executive order that aims to end the practice.