In filings with the Securities and Exchange Commission, both US banks reported having received queries on the issue, days after Barclays Bank, Deutsche Bank, UBS and Royal Bank of Scotland also acknowledged being under investigation over foreign exchange market issues.
"Government agencies in the US and other jurisdictions are conducting investigations or making inquiries regarding trading on the foreign exchange markets," Citi said.
"Citigroup has received requests for information and is cooperating with the investigations and inquiries and responding to the requests."
"These investigations are in the early stages and the firm is cooperating with the relevant authorities," it said.
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Neither bank gave any other information on the direction of the probe.
Yesterday Barclays Bank reported it had received enquiries on its foreign exchange trading covering several years through August 2013.
Before that, the other banks acknowledged they had been contacted.
Britain's Financial Conduct Authority said last month that alongside several other agencies it was investigating a number of firms relating to trading on the foreign exchange market.
The banking sector has already been shaken by a rigging scandal related to the Libor, a benchmark rate for lending between banks that also determines numerous financial and interest rate contracts around the world.
That scandal has already resulted in more than USD 3.5 billion in government settlements with financial institutions, as well as ongoing criminal prosecutions of several traders involved.