Immigrant advocacy groups sued the United States government on Wednesday, alleging that people fleeing persecution are frequently turned away at border crossings with Mexico when they seek asylum or express fear of returning to their homeland.

Reports of people being denied an opportunity to seek asylum have grown, but the lawsuit filed in federal court in Los Angeles pushes the issue into a courtroom and could have far-reaching consequences if a judge grants class-action status.

The American Immigration Council and others say Customs and Border Protection has blocked access to the asylum process for hundreds of people at border crossings in California, Arizona and Texas since last year, violating federal law and international treaty obligations.

OPINION: The long reach of the US border

They say US authorities have threatened to take away children from parents who pursue asylum claims and given incorrect explanations, including that the US no longer accepts asylum seekers from Mexico and Central America or mothers with children.

Customs and Border Protection, which manages border crossings, said it won't comment on pending litigation.

Anyone who expresses fear of returning home at US border crossings is referred to an immigration judge after clearing an initial screening with an asylum officer.

The lawsuit says border inspectors "consistently and systematically" denied that access starting last summer and that the practice continues despite complaints filed in January with the Department of Homeland Security's internal watchdog and civil rights and civil liberties office.

"The overwhelming amount of evidence that we've seen - and it continues to come - really persuaded us that we had to do something more," said Melissa Crow, the American Immigration Council's legal director.

The lawsuit provides accounts of three Mexicans and three Hondurans, identified by pseudonyms, who were allegedly turned away. Five are women affected by drug and gang violence who are living in the Mexican border city of Tijuana, across from San Diego.

READ MORE: Trump's border wall explained

The lawsuit detailed the case of a woman from Mexico whose brother-in-law was a high-ranking police official killed and dismembered by a drug cartel in Mexico, which then threatened her family.

When she arrived with her children at the San Ysidro, California, port of entry and said she was afraid to go back, a border officer coerced her into signing a form in English that she did not understand withdrawing her application for admission to the US, the lawsuit said.

Apprehensions of immigrants on the southwest border have dropped since the beginning of the Trump administration in January, according to US Customs and Border Protection statistics.

Apprehensions are down more than 67 percent to more than 21,000 in June of 2017 from more than 66,000 in October.