WASHINGTON —  The Bush administration is considering an agreement with Mexico that would add thousands of Mexicans working legally in the United States to the Social Security system, making them eligible for millions of dollars in benefits.

"This is an issue that is being explored on a technical level," White House spokeswoman Claire Buchan said Thursday. "No decisions have been made."

Such an agreement would not be unusual. The United States already has 20 existing pacts with other countries, ranging from Canada to South Korea. The Social Security Administration pays 94,022 beneficiaries from other countries an average of $162 a month, for a total of $184 million a year.

An agreement with Mexico could add 162,000 beneficiaries in the first five years, according to The Washington Post, which first reported the story in Thursday's editions, citing an anonymous House Republican aide. The total cost could be as much as $1 billion a year.

Jim Courtney, a spokesman for the Social Security Administration, said the agency had no estimates on cost or how many people would be affected.

"There aren't numbers yet because our actuaries continue to do the research," he said. "There are no preliminary numbers. The numbers are a work in progress."

But concern is growing on Capitol Hill that any agreement with Mexico would add a huge burden to the Social Security system, which already is facing big shortfalls in the next 15 to 20 years.

"We are concerned about the sheer magnitude of the agreement," the House Republican aide said.

Nearly 46 million people currently receive $372 billion in Social Security benefits.