US asylum officers slammed Donald Trump's policy of forcing migrants to remain in Mexico while they await immigration hearings in the United States, urging a federal appeals court on Wednesday to block the administration from continuing the programme.The officers, who are directed to implement the policy, said it is threatening migrants' lives and is "fundamentally contrary to the moral fabric of our Nation."The labour union representing asylum officers filed a friend-of-the-court brief that sided with the American Civil Liberties Union and other groups challenging Mr Trump's Migrant Protection Protocols (MPP) programme, which has sent 12,000 asylum-seeking migrants to Mexico since January.The policy is an effort to deter migrants from seeking entry into the United States and to keep them out of the country while courts consider their claims.The union argued that the policy goes directly against the nation's long-standing view that asylum seekers and refugees should have a way to escape persecution in their homelands, with the United States embracing its status as a safe haven since even before it was founded - with the arrival of the Pilgrims in the 17th century.The union said in court papers that the policy is compelling sworn officers to participate in the "widespread violation" of international and federal law - "something that they did not sign up to do when they decided to become asylum and refugee officers for the United States government.""Asylum officers are duty bound to protect vulnerable asylum seekers from persecution," the American Federation of Government Employees Local 1924, which represents 2,500 federal workers, including asylum officers, said in a 37-page court filing with the 9th Circuit Court of Appeals in California. "They should not be forced to honour departmental directives that are fundamentally contrary to the moral fabric of our Nation and our international and domestic legal obligations."The legal filing is an unusual public rebuke of a sitting president by his own employees, and it plunges a highly trained officer corps that typically operates under strict secrecy into a public legal battle over one of Mr Trump's most prized immigration policies.Under Mr Trump, the asylum division has become a target of internal ire, often assailed for approving most initial asylum screenings and sending migrants to immigration court for a full hearing. Trump administration officials have said most cases are ultimately denied.Last week, the acting director of US Citizenship and Immigration Services, Ken Cuccinelli, outraged some asylum officers by sending a staff email they felt criticised them for approving so many initial screenings.Mr Trump placed Mr Cuccinelli, an immigration hardliner, in the position, ostensibly as part of his move to get tough on immigration policy, and the union's legal filing appears to be directly at odds with that approach.The policy has been challenged in federal court, with a lower-court judge temporarily halting the MPP in April, saying it probably violates federal law. A three-judge panel on the 9th Circuit allowed the programme to resume in May while the court considers the policy's legality.Justice Department lawyers have said in court filings that migrant families are filing thousands of sham claims because they virtually guarantee their release into the United States pending a hearing in the backlogged immigration courts. The US government cannot process the migrants' cases quickly or detain children for long periods, which means some migrants can stay in the country for months or years while waiting for their cases to play out.Ending the programme, the government lawyers have said, "would impose immediate, substantial harm on the government's ability to manage the crisis on our southern border."The Justice Department declined to comment on the filing on Wednesday night.The Department of Homeland Security, which oversees the programme, did not respond to a request for comment.The influx of Central American migrants at the southern border has overwhelmed the US immigration system. It has also led to a political fight between congressional Democrats and the White House regarding conditions in crowded border holding facilities amid Mr Trump's push for heightened enforcement.More than 144,000 migrants were taken into custody in May after crossing the southern border, the largest monthly total in more than a decade, and asylum filings have soared.Trump administration officials this week have been pleading with Congress to approve emergency funding for the humanitarian crisis at the border. The Senate responded on Wednesday, passing a $4.6bn (£3.6bn) emergency spending measure amid debates about treatment of migrants and the risks they face as they try to enter the United States, with a graphic photo of a migrant and his young daughter having drowned in the Rio Grande as the backdrop.In the federal court filing, the asylum officers said they are enforcing the laws as Congress intended, based on approaches and international treaties shaped after World War II and atrocities connected to the Holocaust.Federal laws hinge on the principle of "nonrefoulement" - which means people should not be sent back to countries where they could be harmed or killed. To qualify for asylum, migrants must show that they face harm based on their "race, religion, nationality, membership in a particular social group or political opinion."The asylum officers have said Mexico is too dangerous for Central American asylum seekers, particularly for women, for people who are gay, lesbian or transgender, and for indigenous minority groups. They cited State Department reports showing that gang violence and activity is widespread and that crimes are rarely solved."Mexico is simply not safe for Central American asylum seekers," the filing said, noting that gangs that terrorised migrants in their home countries might easily follow them into Mexico. "And despite professing a commitment to protecting the rights of people seeking asylum, the Mexican government has proven unable to provide this protection."Asylum officers said that the US asylum system is "not, as the Administration has claimed, fundamentally broken," and that they could handle more cases quickly without sending people back to Mexico.The MPP is "entirely unnecessary, as our immigration system has the foundation and agility necessary to deal with the flow of migrants through our Southern Border," the officers said in the filing.The officers said they fear that the MPP is sending asylum seekers back to a country where they are in danger, a violation of federal and international law. They added that immigration agents do not ask migrants if they fear persecution or torture in Mexico, and that they only send migrants to asylum officers for screenings if the migrants independently express fear of return.The latter are granted an initial asylum screening, often by phone or video. But they must prove that they are "more likely than not" going to face persecution in Mexico, a higher bar than in the immigration courts, where migrants are offered safeguards such as access to lawyers, a reading of their rights, and the right to appeal."The MPP, however, provides none of these safeguards," the officers said.Officials are attempting to extend the programme along the nearly 2,000-mile border and are giving Mexico time to expand its shelter capacity, a top official at US Customs and Border Protection has said.The Washington Post