It’s hard for the spring housing market to heat up when for-sale signs in the region are so few and far between.

Sales of single-family homes and condominiums each tumbled 24 percent in May from a year ago, the Greater Boston Association of Realtors reported on Tuesday.

It was the slowest May for condo sales in the area since 2015, with the exception of 2020, when there were few open houses due to the pandemic, according to the realtors’ data. Deals for single-family homes were the weakest in May since 2011, except for 2020.

Most sales that closed last month went under contract in March and April, usually a time when the market emerges from winter hibernation. Instead, it’s been an especially chilly spring selling season.

The average rate on a 30-year fixed mortgage was 6.5 percent from March through the end of May, compared with 4.7 percent during the same period last year, national figures from Freddie Mac show.

With many homeowners holding low-rate mortgages, it’s understandable why they wouldn’t want to sell now. Moreover, with inventories tight and prices high, moving within the region is a daunting prospect.

Despite higher mortgage costs, would-be buyers are plentiful, agents say. The problem, as has been the case since before the pandemic, is inventory — the extreme lack of it. New listings of houses and condos were down 13 percent and 12 percent, respectively, in the 64 towns inside of Interstate 495 covered by the Greater Boston realtors.

The relatively few houses that hit the market were snapped up within two weeks, while condos took a few days more to sell.

The imbalance between supply and demand continues to push prices higher, albeit it a slower rate than last year.

In May, the median price of single-family homes reached a record $900,000, an increase of 2.9 percent from a year earlier. The median condo sold for $726,000, up 3.7 percent and also a new high. (The median price is the point at which half of sales are more expensive and half are less.)

“Although sales volume remains light by historic standards, we’ve seen a steady increase in buyer interest and urgency as the spring has gone on,’’ said Alison Socha, president of the Greater Boston realtors group and an agent with Leading Edge Real Estate in Melrose.

Overall inventories improved in May. Active listings of single-family homes rose 9.7 percent from April and 4.6 percent from May 2022. Condo listings were up 7.4 percent from last month but were down 0.9 percent from a year ago.

“Unfortunately, we haven’t had nearly enough listings to meet the increased demand,’’ ­Socha said in a statement.

Statewide, the median price of a single-family house dropped 2 percent on an annual basis to $589,000 in May, the second consecutive monthly drop, ­according to a separate report on Tuesday from The Warren Group, a real estate data firm. The typical condo sold for $526,000, up 0.2 percent.

Similar trends for sales and prices are unfolding across the country.

Tom Lawler, a housing economist writing in the Calculated­Risk real estate blog, estimated that single-family homes sales fell 21 percent in May from a year earlier, while the median price declined 0.5 percent.

In more than three dozen US markets tracked by Calculated­Risk’s Bill McBride, inventory rose an average of 20 percent, while active listings fell by the same amount.

It’s been a raw spring for house hunters in Massachusetts. But if inventories continue to move higher, sales might heat up in the summer.

Larry Edelman can be reached at larry.edelman@globe.com.