NEW YORK – Home ownership may be falling out of reach for more Americans as lenders toughen their standards for Federal Housing Administration-insured loans beyond what the agency itself requires.

Mortgage lenders such as Wells Fargo and Bank of America, the two largest, have raised the minimum credit score on FHA-insured loans that they will buy to 640 from 620. About 6.3 million people fall within that range, according to FICO, which created the formula for the ratings.

The higher hurdles for FHA loans, used in about one-fifth of U.S. home purchases, add to challenges for a housing market already struggling with record-low sales and surging foreclosures. Although lax lending fueled the bust that led to the recession, the new requirements will stifle the real estate recovery needed to revive the economy, said Ron Phipps, president of the National Association of Realtors.

“We’ve gone from silly to stupid,” Phipps, principal partner of Phipps Realty Inc., said in a telephone interview from his home in Warwick, R.I. “People who should be getting credit can’t get it. To have a healthy real estate market, you need activity. You need transactions.”

The FHA, which previously didn’t have minimums for FICO scores, began in October to require grades of at least 500, and more than 580 for loans with down payments of as little as 3.5 percent. Borrowers with scores between those levels must put 10 percent down. Several lenders moved minimums to about 620 at the start of 2009, the companies said then.

FICO scores range from 300 to 850. The grades are based on data such as whether borrowers have missed debt payments, balances on their credit cards relative to borrowing limits, and the length of their credit history, meaning consumers who’ve never fallen delinquent can have lower scores, according to the company’s website.

Requiring a 640 credit score excludes as much as 15 percent of FHA borrowers, David Stevens, the agency’s commissioner, said in an interview. Minorities and borrowers in communities hit hardest by the recession are most likely to lose based on FICO scores, he said.

“We are restricting opportunity and access for those who can least afford it,” Stevens said. “We need to find a better way to provide access to these families who are being cut out simply because lenders are putting arbitrary overlays on top of our requirements.”

FHA insurance covers lenders or debt investors when borrowers default. One of every five home purchases has relied on the loans in the fiscal year through July. They accounted for one-third of purchases by first-time homebuyers in the year ended Sept. 30.

Mortgage companies are tightening FHA standards partly because of the higher costs they face in servicing delinquent loans, said Luke Hayden, president of the mortgage unit of Mount Laurel, N.J.-based PHH Corp. keeping defaults low, they can also boost the prices they fetch for bonds filled with the loans, and thus offer lower rates, he said.