(CNN) The union representing US asylum officers asked a federal court to end the Trump administration's policy of returning some asylum seekers to Mexico for the duration of their immigration hearing.

"Asylum officers are duty bound to protect vulnerable asylum seekers from persecution. However, under the (Migrant Protection Protocols), they face a conflict between the directives of their departmental leaders to follow the MPP and adherence to our Nation's legal commitment to not returning the persecuted to a territory where they will face persecution," they wrote in a court filing.

"They should not be forced to honor departmental directives that are fundamentally contrary to the moral fabric of our Nation and our international and domestic legal obligations."

Ken Cuccinelli, the acting director of US Citizenship and Immigration Services, defended the policy in a series of tweets Wednesday evening, saying: "This lawsuit is an attempt by the union to score short-term political points. Our ability to provide humanitarian relief to those who lawfully qualify for it will collapse under this influx of fraudulent and non-meritorious claims."

He later added : "MPP protects both the vulnerable, while they wait for their hearing, and our asylum system from choking on meritless claims. THAT is what protection officers signed up for."

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