The United States announced sanctions on Thursday against Havin Bank LTD, a London-based Cuban entity also known as Havana International Bank, dealing a blow to the Cuban financial system.
The Treasury Department said it had placed the bank on its Office of Foreign Assets Control black list.
The bank has been in operation since August 1973, and is the only one with entirely Cuban capital outside Cuba. Its principal shareholder is the Central Bank of Cuba.
From its perch in London, the bank maintains a network of 400 correspondents around the world that provide banking services for the Cuban market.
The sanctions come just as the island's Castro family government has allowed the US currency to circulate, permitting some state stores to sell food for dollars. It recently eliminated a 10 per cent tax on dollar transactions.
US President Donald Trump's administration reversed an opening with Cuba initiated by former president Barack Obama, hardening a trade embargo in effect since 1962.
In doing so, it has cited human rights violations by Cuba and Havana's support for the socialist government of Nicolas Maduro in Venezuela.