Asking prices on houses across the US rose again in May as the spring selling season continued to work through excess inventory. House prices rose in 23 of 26 major metropolitan markets, with 22 of these markets showing the third consecutive month of price increases, according to a Real-Time Housing Market Update published Tuesday by Altos Research and Real IQ. Asking prices were down by 3.7% in Las Vegas, 1% in Salt Lake City and 0.7% in Seattle. "With the spring selling season well underway, inventory growth remains quite restrained easing pressure on listing prices," the report's authors wrote. The Altos 10-City Composite Price Index increased by 2.6% during May and is up 4.8% during the most recent three-month period. The 10-city index asking price bottomed in January at $470,017 and has increased every month in '09, up to $497,960 in May, according to the report. At the same time, housing inventory fell by 0.2% in May and 1.7% during the last three months. The number of listed properties fell in 14 of the 26 markets in May. "This illustrates the real estate seasonality in addition to clearing of properties in the low-end of the market, leaving the properties available on the market at a higher price point," researchers said in the report. The 10-city composite averaged 163 days on market in May, down 1.9% from 166 days on market last month. San Francisco experienced the shortest average time-on-market of 96 days. Boston followed in a close second at 100 average days on market. Miami saw the slowest turn time with an average 260 days on market, followed by Chicago at 212 days on market. Write to Diana Golobay.