The Biden administration on Sunday demanded that Texas officials stop preventing federal Border Patrol agents from entering a section of the U.S. southern border commandeered by state National Guard soldiers last week, calling the actions "clearly unconstitutional," according to a letter obtained by CBS News.

If the Texas National Guard does not heed the Biden administration's demand and cease its efforts to block Border Patrol's access to a public park in the border town of Eagle Pass, the Department of Homeland Security will refer the matter to the Justice Department and explore other options, Jonathan Meyer, the top lawyer at DHS, warned in his letter to Texas Attorney General Ken Paxton.

Meyer said Texas's abrupt move last week to seize control of Eagle Pass' Shelby Park, an area where Border Patrol used to inspect and hold migrants, had "impeded" federal operations to apprehend migrants and rescue those who may be in distress. He cited the drownings of three migrants, a woman and two children, near Shelby Park on Friday.

The White House, DHS and Texas Democratic Rep. Henry Cuellar have said Texas National Guard soldiers blocked Border Patrol agents from attempting to render aid to the migrants on Friday. On Sunday, the Texas Military Department, which controls the state National Guard, disputed the claims, saying two of the migrants had already drowned when Border Patrol requested access to Shelby Park to respond to the incident. The department has not denied barring Border Patrol from entering the park. 

"Texas's failure to provide access to the border persists even in instances of imminent danger to life and safety," Meyer said in his letter to Paxton.

DHS gave Texas until the end of the day on Wednesday to relent.

"If you have not confirmed by the end of day on January 17, 2024, that Texas will cease and desist its efforts to block Border Patrol's access in and around the Shelby Park area and remove all barriers to access to the U.S.-Mexico border, we will refer the matter to the Department of Justice for appropriate action and consider all other options available to restore Border Patrol's access to the border," Meyer wrote.

CBS News has reached out to the Texas Attorney General's office for comment.

Texas has said its move to seize control of Shelby Park and ban federal agents from the area through armed soldiers, fencing and razor wire is designed to deter unlawful crossings by migrants. It's the latest flashpoint in a yearslong political and legal feud over U.S. border policy between Republican Gov. Greg Abbott and President Biden, a Democrat.

Over the past three years, Abbott has taken unprecedented and controversial steps in response to what he has deemed to be a lackluster effort by Mr. Biden to contain record levels of migrant crossings. Under his direction, Texas has bused tens of thousands of migrants to Democratic-led cities, arrested thousands of migrant adults on trespassing charges and fortified the Rio Grande with razor wire and floating barriers.

But Texas' effort to physically block federal agents from processing migrants in a public park that covers roughly 2.5 miles of the U.S.-Mexico border is arguably the state's most drastic action yet. 

In his letter on Sunday, Meyer noted the federal government is the one tasked with regulating immigration, including the inspection, processing and arrest of migrants who enter the U.S. without permission. Texas' seizure of Shelby Park is obstructing federal officials from carrying out these obligations, he said. 

"State law cannot be applied to restrain federal agents from carrying out these federally authorized activities," Meyer added.

Meyer also noted that some of the areas Texas had commandeered sit on federal land. He said a 2015 agreement between the city of Eagle Pass, which oversees Shelby Park, and Border Patrol guarantees federal officials access to the park's gates. Some of the barriers used by Texas to seal off the park are owned by the federal government, Meyer added.

Last week, the Justice Department alerted the Supreme Court of Texas' actions in Eagle Pass as part of a case over a lower court ruling that barred Border Patrol from removing the razor wire the state has placed along some sections of the U.S. southern border. The Justice Department is asking the high court to suspend that ruling, saying it prevents Border Patrol from processing and rescuing migrants already on U.S. soil.