Just over a year ago, cash-strapped Venezuela quietly conducted a little-noticed gold-for-cash swap with Citigroup as part of which Maduro converted part of his nation's gold reserves into at least $1 billion in cash through a swap with Citibank.

"With no warning, Citibank says that in 30 days it will close the Central Bank and the Bank of Venezuela's accounts," Maduro said in a speech, adding that the government used the U.S. bank for transactions in the United States and globally.