A new report from Zillow says the amount of homes for sale dropped at the fastest speed in almost four years.

According to the report, there were about 9 percent fewer homes to choose from nationally this May compared to a year ago. That's one reason home prices continue to climb.

"With fewer homes for sale, competition becomes so fierce that people snatch them up quickly, and homes spend less and less time on the market," the report said.

The last time the market saw a major drop was in August 2013 when inventory fell about 10 percent.

Seattle home values rose almost 13 percent in May, while the number of homes for sale dropped a little more than 22 percent, the Zillow report said.

Some reasons for the low inventory include high demand, a lack of new construction, Americans moving from their homes less, more intense competition for buyers, and sellers reluctant to becoming buyers in the fast-paced, high-priced market.

The report also found that U.S. home values rose a little more than 7 percent in the last year to a median value of $199,200.