Home sales continued their slide in June, making the first half of 2008 the slowest housing market in more than 15 years, a Boston real estate publishing firm said today in its monthly housing report.

There were 4,663 sales of single-family homes last month, about a 15 percent decline from the number of sales transactions closed in June 2007. The slowdown drove the median price down 8.6 percent, to $329,000, according to The Warren Group.

During the first half of this year, there were 18,487 home sales — the fewest transactions for the period since 1991.

“What’s clear from these numbers is we haven’t reached the bottom of the market yet,” Warren Group’s chief executive, Timothy Warren Jr., said in a statement. “The rash of bad news about bank failures and Fannie Mae’s and Freddie Mac’s capital needs convinces us that a turnaround isn’t going to happen this year.”

Condo sales were hit even harder, plummeting 23.3 percent to 2,288 last month. But the median condo price rose to $295,000 from $292,000 a year ago.

For the first half of the year, condo sales were down 30 percent compared with 2007, while prices declined 1.1 percent.