(CNN) A federal employee labor union is suing the US government for requiring "essential" employees to work without pay during the partial government shutdown.

The American Federation of Government Employees alleged Monday that the government is violating the Fair Labor Standards Act by forcing employees deemed essential to work without pay.

About a quarter of the government has been affected by the shutdown, including correctional officers, Border Patrol and Immigration and Customs Enforcement agents, and transportation security officers. Across different government agencies, about 380,000 federal employees will be furloughed and 420,000 will work without pay due to the shutdown continuing into the new year.

AFGE filed the suit in the US Court of Federal Claims on behalf of all essential government workers, as well as named plaintiffs Justin Tarovisky and Grayson Sharp, who work for the Bureau of Prisoners.

The union's president, J. David Cox, called the requirement that some federal employees work without pay "inhumane" in a statement Monday.

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