But Russia's access to $630bn (£470bn) of foreign currency reserves has now been frozen.

Credit ratings agencies have warned that a debt default is "imminent".

The International Monetary Fund (IMF) has said while it is concerned about the impact of any Russian debt default, it does not believe this would trigger a global financial crisis.

The Russian government - and firms such as Gazprom, Lukoil and Sberbank - owe about $150bn to overseas investors.

Most of this is in either dollars or euros, and Russian institutions are now barred from accessing dollar or euro assets that are held abroad because of sanctions.

If Russia does default, it will be the country's first debt default since 1998.

It would also be its first default on a foreign-currency debt since the 1917 revolution when the new Bolshevik government refused to recognize the debts of the last tsar.