U.S. home prices rose for the fifth straight month and posted the second quarterly increase, but the pace of appreciation in September slowed and was less than expected, according to Standard & Poor's/Case-Shiller indexes on Tuesday.

The S&P composite index of home prices in 20 metropolitan areas rose 0.3 percent in September from August after a 1.2 percent rise the prior month, below the 0.8 percent rise forecast in a Reuters poll.

The 20-city index had an annual decline of 9.4 percent.

The national index for the third quarter increased 3.1 percent from the prior quarter, the same as in the second quarter, resulting in an 8.9 percent annual drop. That was a significant improvement from the 14.7 percent annual downturn reported in the prior quarter and 19 percent slump in the first quarter.

The 10-city composite index rose 0.4 percent in September after a 1.3 percent August gain. The annual drop was 8.5 percent.

We have seen broad improvement in home prices for most of the past six months, David M. Blitzer, chairman of the Index Committee at S&P, said in a statement. However, the gains in the most recent month are more modest than during the seasonally strong summer months.

(Reporting by Lynn Adler, Editing by Chizu Nomiyama)