WASHINGTON — Existing home sales fell for the third consecutive month in February, showing a modest decline amid concerns that a home-buyer tax credit that helped revive the market last year might not have a similar impact during this spring’s buying season.

Sales of existing houses, townhouses, condominiums and cooperatives fell 0.6 percent to a seasonally adjusted rate of 5.02 percent in February compared with the previous month, according to data from the National Association of Realtors. That was in line with what analysts had expected and a much smaller decline than the market had in December and January.

Bad weather kept some buyers on the sidelines, Lawrence Yun, the group’s chief economist, said. But the market is stronger now than it was last year, he said. According to the industry data, home sales rose 7 percent in February, compared with the same period a year ago.

The market has been helped by historically low interest rates and cheap foreclosed properties on the market. The Realtors’ group estimated that 35 percent of sales last month were of distressed properties. But economists worry that high levels of unemployment will make it difficult for the housing market to make a robust recovery soon.

In February, a pickup in sales in the Northeast and Midwest was offset by declines in the rest of the country. Existing-home sales fell in 4.7 percent in the West and 1.1 percent in the South.

Median home prices fell another 1.8 percent in February to $165,100 compared with the same period last year.

More troubling for economists was the 9.5 percent increase in the housing inventory. It would now take 8.6 months to sell all of the homes on the market at the current sales rate, compared with a 7.8-month supply in January.

“The supply situation is only likely to get worse, too,” Paul Dales, an economist for Capital Economics, said in a research note. Millions of homeowners delinquent on their mortgage are facing foreclosure, which will add to the inventory later.

Yun acknowledged that “the housing recovery is fragile at the moment” and the real estate market is facing a crucial test. The industry has set its sights on this spring’s buying season, when more people could take advantage of a recently renewed tax credit for first-time home buyers and others that is due to expire in April.

“The key test for a durable recovery comes in the next few month as the tax credit deadline approaches,” Yun said.

The $8,000 tax credit for first-time home buyers helped propel sales last year, prompting Congress to extend and expand it. First-time home buyers now have until April 30 to sign a contract for a home and qualify for the $8,000 tax credit. Repeat buyers are eligible for a $6,500 tax credit with the same time restrictions.

But some economists are already raising concerns about whether the renewed tax credit will have a similar impact on the housing market.

“Unfortunately, despite the high hopes associated with the extended and expanded home buyer tax credit, housing activity appears to have faced a setback that went beyond the impact of adverse weather conditions,” Fannie Mae, the mortgage financing company, said in its monthly market analysis.

The credit might have already “dried up” the pool of qualified first-time buyers and the credit being offered to repeat buyers might not be generous enough to spur sales activity, the analysis said.