The program is available, but few graduates are signing up.

Sallie Mae, the nation's largest servicer of federal student loans, is failing to enroll many of its distressed borrowers into one of the Obama administration's main initiatives for alleviating high student debt.

Estimates provided by the White House suggest that Sallie Mae, or SLM Corp., has enrolled relatively few borrowers into the Income-Based Repayment Program. Sallie Mae dominates the now-discontinued Federal Family Education Loan Program, owning between 37 and 40 percent of the outstanding FFELP debt held by the private sector. But its share of FFELP borrowers who are enrolled in IBR is about half that, or 15 to 18 percent.

Officials have cautioned that the government figures are just rough estimates, but so far, they are the only available indicator of how Sallie Mae utilizes the loan relief program.

Outstanding federal student debt has nearly doubled since 2007 to 1 trillion dollars, government data show, while the average borrower with federal student loans now carries more than 26,000 dollars in debt -- a 42.7 percent increase since 2007. The IBR program has emerged as President Barack Obama's signature tool to address this burgeoning crisis.

In May, Obama warned that this debt load "doesn't just hold back our young graduates. It holds back our entire middle class." He added that student loan payments "can last for years, even decades, which means young people are putting off buying their first car or their first house -- the things that grow our economy and create new jobs."

The White House and other federal policymakers have heralded the IBR program as a potential antidote to the risk of student-debt-fueled economic malaise. The program caps monthly payments on federal student loans relative to borrowers' incomes and gives some debtors the opportunity to have their remaining loans forgiven after years of steady payments.