Rating agency Moody's Investors Service has put the US government's AAA bond  on review for a possible downgrade as concerns about debt levels mount.

The agency was prompted into action over fears that the statutory debt limit will not be raised 'on a timely basis', and could lead to a default.

Moody's described the risk of the US missing a payment as 'small but rising risk', adding that any kind of default would mean the AAA rating would not be appropriate.

The rating agency has also put the AAA ratings of lenders Fannie Mae, Freddie Mac, the Federal Home Loan Banks and the Federal Farm Credit Banks.

Securities backed, linked or guaranteed by the US government have also been put on review.

It added, 'While the debt limit has been raised numerous times in the past, and sometimes the issue has been contentious, bond interest and principal have always been paid on time.'

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