United Airlines Inc (UAL.O) is facing claims that it unlawfully denied religious and medical exemptions from a requirement that employees receive COVID-19 vaccines after allegedly making it difficult for workers to apply for them.

Six United employees filed a class action in Texas federal court on Tuesday claiming that workers who sought exemptions from the vaccine mandate were subjected to intrusive inquiries about their medical conditions or religious beliefs, including a requirement that they obtain letters from pastors.

Chicago-based United in a statement said the lawsuit was without merit, and that it has seen an overwhelmingly positive response from employees since announcing its vaccine requirement last month. More than 97% of United's U.S.-based employees are vaccinated, the airline said.

The lawsuit highlights the thorny legal issues faced by employers in mandating vaccines, and comes as President Joe Biden is seeking to require companies with 100 or more employees to ensure their workforces are fully vaccinated or tested for COVID-19 weekly.

The plaintiffs on Wednesday asked the court to temporarily bar enforcement of United's mandate for employees who request exemptions. United has required workers to receive at least the first dose of a vaccine by September 27 or face termination, according to the lawsuit.

The airline has already faced a separate legal challenge to its vaccine mandate, which was dismissed by a U.S. judge in Florida last week. The judge said that lawsuit was not filed properly.

The plaintiffs in Tuesday's lawsuit say United gave employees only until August 31 to request religious or medical exemptions from the vaccine requirement, and that it has automatically denied requests filed after that deadline.

They accused United of violating federal laws prohibiting discrimination based on religion and disability.

The workers are seeking to represent a nationwide class that they said would likely include more than 2,000 United employees.