Shares in the two biggest mortgage lenders in the United States have tumbled, creating fears that they may not have enough funds to cover mortgage-related losses.

At the close of trading on Friday, Freddie Mac shares had lost 3.1 per cent, falling to $7.75 and shares in Fannie Mae were down 22 per cent at $10.25 dollars.

The shares of the two firms have plunged around 75 per cent since the start of the year.

"The problem is is that these two companies have a huge sway over the US mortgage market," Al Jazeera's John Terrett said, reporting from New York.

"They were created really to always make sure there's enough in the pot for Americans to get a mortgage and the problem is, with the credit crunch, there are rumours that they won't be able to pay off ... their eye popping $5 trillion debt."

The size of the mortgage giants means that any hint of trouble could have a widespread effect on the …