Four US financial institutions, including Citigroup, have failed stress tests designed to show they could withstand a financial shock.

The Federal Reserve said Citi, SunTrust, Ally Financial and MetLife failed to show they have enough capital to survive another serious downturn.

Citigroup is the third-largest US bank. The majority of the 19 tested passed.

All those tested are in a much stronger position than they were after the 2008 financial crisis, the Fed added.

The Fed tested the banks' ability to withstand a similar crisis that triggered a rise in unemployment to 13%, a 50% fall in share prices and a 21% drop in house prices.

Their strength is assessed by the amount of "buffer" best-quality assets, known as Tier 1 capital, they would hold if such conditions occurred.

The regulator said Citigroup had a Tier 1 capital ratio of 4.9%, with Ally Financial and SunTrust at 4.4% and 4.8% respectively.

MetLife had 6%, although it was measured slightly differently to the others.