WASHINGTON — The Biden administration on Thursday announced new efforts to limit the flow of unauthorized migrants across the southern border, including the opening of processing centers in the region, while warning that a coming change in the law will not make entering the United States easier.

The announcements came two weeks before the scheduled court-ordered lifting of Title 42, a public health rule issued during the pandemic that gives U.S. officials unusual powers to quickly expel migrants who cross the border without permission. Biden officials fear the change in law will attract a wave of migrants at a time when Republicans are accusing President Biden of being too tolerant of illegal immigration.

In a joint appearance at the State Department on Thursday, Secretary of State Antony J. Blinken and Homeland Security Secretary Alejandro N. Mayorkas said that would-be migrants should not mistake the expiration of Title 42 on May 11 for a green light to cross the border, despite what they called a blitz of misleading information from human smugglers offering them transit.

“The smugglers’ propaganda is false,” Mr. Mayorkas said. “Our border is not open, and will not be open after May 11.”

Mr. Mayorkas and Mr. Blinken coupled those admonitions with the announcement of new measures aimed at reducing the number of people willing to risk the often life-threatening journey north to the U.S.-Mexico border through jungles and desert.

The centerpiece of the effort will be what U.S. officials called “regional processing centers,” designed to determine the eligibility of people to enter the U.S. legally before they begin to travel and direct them to federal programs that serve refugees and other legal migrants.

U.S. officials said the Biden administration would significantly raise admissions to its refugee resettlement program and other pathways to U.S. residency, including family reunification and labor programs, but did not provide further detail.

The first centers will open soon in Colombia and Guatemala, officials said, with more in the planning stage. People seeking entry into the United States can schedule appointments at the centers, which will be staffed with U.S. immigration officials partnering with international organizations, Mr. Mayorkas said.

“The whole model is to reach the people where they are, to cut the smugglers out, and to have them avoid the perilous journey,” Mr. Mayorkas said. Canada and Spain have agreed to accept some lawful migrants referred by the processing centers, officials said.

Mr. Blinken detailed other steps the United States was taking, including a surge of assistance to countries in the region to crack down on human smuggling.

Mr. Mayorkas also said he had directed aides to added more legal pathways for El Salvadorans, Hondurans and Guatemalans to reunite with family in the United States. That program allows a U.S. citizen or permanent resident to apply inside the United States for a family member to come and join them on a “parole” basis before their visa is approved.

But the tone of the remarks by Mr. Mayorkas in particular was stern, as he sought to dispel any notions that the expiration of Title 42 — which a federal court ordered to be lifted after it was challenged by multiple lawsuits — would make entry into the country more feasible.

Mr. Mayorkas stressed that the Biden administration would aggressively exercise other powers to ensure that illegal entries remained difficult for migrants and easy for U.S. officials to reverse.

Another section of U.S. law known as Title 8, Mr. Mayorkas said, carries “stiff consequences for irregular migration,” including a five-year ban and potential criminal charges for people repeatedly caught trying to enter the country. The shift to those authorities will be “swift and immediate” after the expiration of Title 42, Mr. Mayorkas said.

The administration is also completing new rules that would make migrants who failed to apply for humanitarian protection in a country on the way to the U.S. border ineligible for asylum in the United States.

He also said the administration can only do so much, and implored Congress to pass “desperately needed reform to our immigration and asylum system.”

Kevin Appleby, interim director of the Center for Migration Studies of New York, called the regional processing initiative “a positive step forward in ensuring that persons fleeing persecution receive protection.”

He added that the details of implementation would determine its success, and “should not be used to interdict asylum seekers from reaching safety or become window dressing to mask the denial of asylum to individuals at our southern border.”

Asked about the administration’s plans for families apprehended at the border, Mr. Mayorkas said the administration had “no plan to detain families,” and that they might face “expedited removal,” as well as “alternatives to detention” which he did not detail.

The New York Times previously reported the administration considered reviving the practice of detaining migrant families who cross the border illegally — the same policy the president shut down over the past two years because he wanted a more humane immigration system. But the plan was met with widespread backlash from Democrats.

In a statement, the International Refugee Assistance Project said it “strongly opposes” the new measures “as a trade-off for limiting the legal rights of people seeking asylum in the United States.”

The group said it was troubled that the administration was simultaneously pursuing other immigration restrictions, including what opponents call an “asylum ban,” which would substantially limit the number of migrants who could apply for asylum in the United States.

Zolan Kanno-Youngs contributed reporting.