WASHINGTON — The Biden administration announced on Thursday it would offer humanitarian relief to Ukrainians who have been living in the country without legal documentation since March 1 or earlier, signaling additional support for citizens of Ukraine as Russia advanced in the south of the country.

Canada announced similar relief on Thursday, as did the European Union, which said it would offer three years of protection for Ukrainian refugees.The United Nations predicted that 10 million Ukrainians, about a quarter of the country’s population, could be displaced by the Russian invasion. Already, a million people have fled the country as refugees.

Lawmakers and advocacy groups have been calling for the relief, known as temporary protected status. Some have estimated that 28,000 to 30,000 Ukrainians could be eligible for it; the designation gives them permission to stay and work in the United States for 18 months. Often, temporary protected status is extended. In this case, it will not apply to any Ukrainians who entered the country after March 1.

“Russia’s premeditated and unprovoked attack on Ukraine has resulted in an ongoing war, senseless violence and Ukrainians forced to seek refuge in other countries,” Alejandro N. Mayorkas, the homeland security secretary, said in a statement.

The Biden administration moved quickly on the designation, even as advocates have been pleading with the government for months to offer similar relief to people from Cameroon, who have faced violence and persecution when they were not granted asylum in the United States and deported.

Daniel Tse, the founder of the Cameroon Advocacy Network, said he saw a racial component to the decisions.

“It is evidence of anti-blackness and discrimination toward Black immigrants,” Mr. Tse said on Thursday, echoing a growing concern among immigration advocates and some lawmakers.

The Department of Homeland Security, which decides when to grant temporary protected status in close coordination with the White House, did not immediately respond to a request for comment. Previously, Mr. Mayorkas has said that the immigration system treats all migrants equally.

Both Republican and Democratic lawmakers have called for the administration to offer the benefit to Ukrainians.

“Ukrainian Nationals currently in the United States should not be forced to return to Ukraine while the nation is at war with Russia,” Sen. Rob Portman, Republican of Ohio, said on Twitter. “Granting them Temporary Protected Status is the right thing to do.”

Senator Bob Menendez, Democrat of New Jersey and the chairman of the Senate Foreign Relations Committee, praised the administration for giving this relief to Ukrainians. But he also urged the White House to designate citizens of Cameroon, Ethiopia and Afghanistan as eligible for the same benefit.

“Temporary Protected Status was created by Congress for exactly this purpose — to protect people whose home countries have experienced armed conflict, an environmental disaster or extraordinary conditions that prevent people from safely returning home,” Mr. Menendez said in a statement. The program went into effect in 1990.

The administration also paused deportations on commercial flights to the region, according to an official who spoke on condition of anonymity because they were not authorized to speak publicly. On March 1, three Ukrainians were scheduled for deportation to Kyiv on a commercial flight, the official said. Those three people are now eligible to apply for temporary status.

More than 1,000 Ukrainians were caught crossing the southwest border of the United States from October to January, a significant increase compared with previous years, according to government data.

There are currently more than 400,000 people living in the United States under temporary protected status, including immigrants from Burma, El Salvador, Haiti, Honduras, Nepal, Nicaragua, Somalia, South Sudan, Sudan, Syria, Venezuela and Yemen.

Advocates for limiting immigration have been critical of the program, which they say ultimately lets people who receive the designation stay in the United States permanently.

The federal agency that handles applications for the temporary benefit reports about a six-month wait period. Ukrainian immigrants who qualify for the program can also apply for authorization to work in the United States.

On Thursday afternoon, Prime Minister Justin Trudeau of Canada announced a new program for fleeing Ukrainians to be granted quick, temporary immigration status in the country, which has a large Ukrainian population.

His government also announced a special program, through which Ukrainian-Canadians can sponsor family members fleeing Ukraine to settle permanently in Canada.

“Ukrainian immigrants have helped build this country, and we stand with the courageous people of Ukraine in upholding the values that our countries share,” the Canadian government’s news release said.

Catherine Porter contributed reporting from Toronto.