The number of pending home sales in the United States experienced a year-over-year decrease for the first time in two years in May, according to the National Association of Realtors.

In its latest update of the Pending Home Sales Index, the organization said the figure for the month stood at 110.8. This was down 3.7 percent from April's PHSI of 115 and 0.2 percent from the May 2015 figure of 111.

The Pending Home Sales Index takes into consideration all transactions where a contract has been signed but the sale has not closed. The closing usually takes place within one or two months of the contract signing.

The National Association of Realtors says the PHSI usually represents about 20 percent of existing home sales in the United States. A figure of 100 is equal to sales activity in the year 2001, when existing home sales fell within the normal range of 5 million to 5.5 million during the year.

Pending sales fell from the previous month in all four major geographic regions identified by the organization. May's Pending Home Sales Index was the third highest in the past year, but was down from the previous year's level for the first time since August 2014.

"With demand holding firm this spring and homes selling even faster than a year ago, the notable increase in closings in recent months took a dent out of what was available for sale in May and ultimately dragged down contract activity," said Lawrence Yun, chief economist at the National Association of Realtors. "Realtors are acknowledging with increasing frequency lately that buyers continue to be frustrated by the tense competition and lack of affordable homes for sale in their market."

The PHSI for the Northeast stood at 93 in May, down 5.3 percent from April to the same level as the previous year. The Midwest had the next most significant decline from April, with pending sales falling 4.2 percent to an index of 108 – 1.8 percent below that of May 2015.

The South was the only region with year-over-year growth in pending sales, which climbed 0.6 percent from May 2015 to a PHSI of 126.6. However, they were also down 3.1 percent from April. In the West, the index fell 3.4 percent from the previous month and 0.1 percent from the previous year to 102.6.

Yun said rising home prices and a lack of homes for sale are making it difficult for buyers to find an affordable property in some areas. The median price for an existing home in May was $239,700, exceeding a record high of $236,300 set in June 2015. There were 2.15 million existing homes for sale at the end of May, down 5.7 percent from the previous year; May 2015 was also the last time the National Association of Realtors recorded a year-over-year increasing in housing inventory.

"Total housing inventory at the end of each month has remarkably decreased year-over-year now for an entire year," said Yun. "There are simply not enough homes coming onto the market to catch up with demand and to keep prices more in line with inflation and wage growth."

Yun expects that the United Kingdom's decision to leave the European Union will have a mixed impact on the U.S. housing market.

"In the short term, volatility in the financial markets could very likely lead to even lower mortgage rates and increased demand from foreign buyers looking for a safer place to invest their cash," he said. "On the other hand, any prolonged market angst and further economic uncertainty overseas could negatively impact our economy and end up tempering the overall appetite for homebuying."

The National Association of Realtors is currently projecting that there will be 5.44 million existing home sales in the United States this year, up 3.7 percent from 2015. The median growth in home prices, which hit 6.8 percent last year, is expected to moderate to 4 to 5 percent in 2016.