The annual pace of existing home sales hit its strongest point in more than nine years in May, according to the National Association of Realtors. The organization says the limited availability of homes for sale also contributed to an all-time high for median home prices.

The sales of existing homes—defined as completed transactions for single-family homes, townhouses, condominiums, and co-ops—rose 1.8 percent from April's downwardly revised figure to a seasonally adjusted annual rate of 5.53 million. This pace was up 4.5 percent from May 2015 and at its highest level since February 2007, when the annual pace of existing home sales was 5.79 million.

Lawrence Yun, chief economist at the National Association of Realtors, said low mortgage rates are continuing to motivate some buyers to purchase a home. However, he said most of the transactions involve existing homeowners who have decided to downsize or trade up after gaining equity due to rising home prices in recent years.

"With first-time buyers still struggling to enter the market, repeat buyers using the proceeds from the sale of their previous home as their down payment are making up the bulk of home purchases right now," said Yun. "Barring further deceleration in job growth that could ultimately temper demand from these repeat buyers, sales have the potential to mostly maintain their current pace through the summer."

Thirty percent of existing homes purchased in May were bought by first-time buyers, matching the average share of first-time buyers for 2015 as a whole. This share was down from both April and May 2015, when first-time buyers made up 32 percent of existing home purchases.

The median sales price for an existing home increased for the 51st consecutive month in May, reaching $239,700. This price was up 4.7 percent from May 2015 and exceeded the previous peak of $236,300, set in June 2015.

Existing homes were being sold at the fastest pace since the National Association of Realtors began tracking this statistic in May 2011. The typical property was listed for 32 days, seven days faster than April and eight days faster than in May 2015. Nearly half of these homes—49 percent—sold in less than a month.

There were 2.15 million existing homes for sale in May, an increase of 1.4 percent from April. However, this inventory was still 5.7 percent below May 2015 levels, when 2.28 million homes were available for purchase.

"Existing inventory remains subdued throughout much of the country and continues to lag even last year's deficient amount," said Yun. "While new home construction has thankfully crept higher so far this year, there's still a glaring need for even more, to help alleviate the supply pressures that are severely limiting choices and pushing prices out of reach for plenty of prospective first-time buyers."

Mortgage rates remained low, with Freddie Mac reporting an average commitment rate of 3.6 percent for a 30-year conventional fixed rate mortgage. This rate was down from 3.61 percent in April and the lowest seen since May 2013, which had an average rate of 3.54 percent.

Individual investors accounted for 13 percent of existing home sales in May, the same share as in April but a decrease of 1 percentage point from a year ago. Sixty-three percent of investors paid cash for their purchase. Twenty-two percent of all existing homes in May were bought with cash, a decrease of 2 percentage points from both the previous month and previous year.

Distressed sales made up 6 percent of the transactions, down from 10 percent in May 2015 and 7 percent in April. Five percent were foreclosures and 1 percent were short sales. Foreclosures spent a median period of 51 days on the market, while short sales were listed for a median of 103 days.

The average discount on a foreclosure purchase was 12 percent under market value, down from 17 percent in April. The average discount on a short sale purchase rose from 10 percent in April to 11 percent in May.

The Northeast had the strongest year-over-year growth in existing home sales, which rose 11.6 percent to an annual rate of 770,000. This pace was also up 4.1 percent from April. However, the region was also the only one of the National Association of Realtors' four geographic zones to see a decrease in the median sales price of existing homes; this fell 0.1 percent from May 2015 to $268,000.

The Midwest was the only region where existing home sales were down from April. The annual rate for May stood at 1.3 million, down 6.5 percent from the previous month but up 3.2 percent from May 2015. The median price in this region was $190,000, a year-over-year increase of 4.8 percent.

In the West, the annual existing home sales rate of 1.18 million was down 1.7 percent from May 2015 but up 5.4 percent from April. The median price in the region rose 7.7 percent to $346,900.

The South had an annual existing home sales rate of 2.28 million, an increase of 4.6 percent from the previous month and 6.5 percent from the previous year. The median price in the region was $211,500, up 5.9 percent from May 2015.

Single-family home sales made up the majority of existing home transactions, increasing 1.9 percent from April to a seasonally adjusted annual rate of 4.9 million. This pace was also 4.7 percent above the rate of May 2015. The median price for a single-family home was $241,000, a year-over-year increase of 4.6 percent.

Sales of existing condominiums and co-ops increased 3.3 percent from May 2015 and 1.6 percent from April to a seasonally adjusted annual rate of 630,000 units. The median price for these units was $229,600, a 6 percent increase from the previous year.