Us second-hand housing signings fell unexpectedly in April, the third decline in the past four months, suggesting a lack of affordable housing that continues to constrain the housing market.

The index of second-hand housing contracts fell 4.4% month-on-month to 106.2, the lowest since may last year, according to data released by the national association of real estate brokers on Thursday. The median forecast in a Bloomberg survey is a rise of 0.4%.

The higher prices demanded by sellers reflect the limited supply of housing, which also makes prices more and more beyond the affordability of buyers, limiting sales. However, the number of second-hand housing contracts increased by 53.5% year-on-year on an unadjusted basis in April.

This suggests that once the number of houses increases, potential interest in buying homes may boost sales.

Lawrence Yun, the association's chief economist, said supply was expected to improve as early as this fall. The rising price of sellers has caused some bidding wars, which also brings questions about the risks of the real estate market.

JPMorgan ChaseJamie Dimon, chief executive, said at a congressional hearing earlier this week that the housing market was "a little frothy" but that banks had more capital than they did during the financial crisis. Morgan StanleyJames Gorman, chief executive, says there is no real estate bubble across the country, but some suburbs look like there are bubbles.