WASHINGTON — Americans stepped up their home-buying for a third straight month in July, as sales accelerated to the strongest pace in eight years.

The National Association of Realtors said Thursday that sales of existing homes rose 2 percent last month to a seasonally adjusted annual rate of 5.59 million, the fastest rate since February 2007. Sales have jumped 9.6 percent over the past 12 months, while the number of listings has declined 4.7 percent.

Steady job growth and relatively low mortgage rates have convinced current homeowners to purchase homes, while first-time buyers remain scarce. The housing market contains a mere 4.8 months’ supply of homes, meaning that prices are rising for an increasingly narrow set of properties.

The slow, six-year recovery from the Great Recession has finally revitalized the housing market. Home sales have soared in recent months, as more current homeowners have returned to the real estate market for an upgrade or to downsize as they approach retirement. Yet the upswing also reflects increasing problems with affordability that have left first-time buyers on the sidelines.

“When first-time homebuyers compete with people who are more qualified borrowers that have additional cash, they tend to lose,” said Budge Huskey, chief executive of the real estate brokerage Coldwell Banker.

The median home price climbed 5.6 percent over the past 12 months to $234,000. Just 28 percent of the purchases last month went to first-time homebuyers, a group that historically accounted for 40 percent of sales. A more balanced market would contain six months’ of supply – instead of less than five – and provide potential homebuyers with a greater selection.

Current homeowners with equity have been able to absorb some of that price appreciation as they’ve shopped for another home. But the recent sales explosion also reflects two critical factors: the economy adding a solid 2.9 million jobs over the past 12 months, and the average, 30-year fixed mortgage rate staying around 4 percent. At roughly two percentage points below the historical level, mortgage rates have reduced monthly borrowing costs for buyers.

Still, the trajectory of mortgage rates – and sales – going forward is unclear. It’s possible that a weakening global economy will cause more investors to buy U.S. Treasury bonds, a move that has historically held down mortgage rates. The average mortgage rate has slipped slightly as China has endured stock market volatility and reduced the value of its currency.

Yet the Federal Reserve is preparing to raise a key interest rate for the first time in nearly a decade. Economists say the Fed could lift its fed funds rate from near-zero as soon as September, an increase that would potentially cause mortgage rates to rise. When Fed officials previously announced plans in 2013 to pull back on other forms of economic stimulus, mortgage rates suddenly spiked and derailed home sales for several months.

The July sales increase occurred in the South and West. Home purchases remained unchanged in the Midwest and slipped in the Northeast.

Despite the higher sales, more Americans are choosing to rent. So far this year, the share of the U.S. population who owns homes has fallen to 63.4 percent, a 48-year low, according to the Census Bureau. The shift toward renting has reduced vacancies and caused rental prices to increase at 4.3 percent, more than double the meager 2.1 annual increase in average hourly wages.

Fewer millennials younger than 34 are forming households and the purchase of first homes is increasingly deferred. The median first-time buyer is renting longer and spending more money to purchase their homes, according to the real estate firm Zillow. Meanwhile, the percentage of young adults living with their parents increased in 2015, according to the Terner Center for Housing Innovation at the University of California, Berkeley.