Former Federal Reserve Chairman Alan Greenspan said on Monday he is concerned about the fallout from a U.S. housing bubble but added it's unclear whether that might cause a broader upheaval in the global economy.

"We, unlike the rest of the world, are showing some modest price declines," Greenspan said in an interview with CNBC Television.

But when asked whether that will spark a broader economic turndown, he said "the problem is that we haven't seen one yet, and I don't know, but I will say that is a critical question."

Greenspan said the Fed tried to raise mortgage rates in 2004 to weigh against an emerging bubble but was unsuccessful.