A glut of vacant homes suggests that the U.S. housing market has not yet stabilized and may be poised for another downturn, Merrill Lynch said in a research note released on Monday.

"Now that oil prices and mortgage rates have stopped falling, we will be back lamenting the downturn in the housing market and its spreading effects on the economy in the second quarter, much as we were in the summer and fall 2006," Merrill Lynch economist David Rosenberg wrote. "Looking at the inventory backlog and still-stretched affordability levels, this story is far from over."