Citigroup, the largest U.S. bank, may need more capital from outside investors despite raising billions of dollars after reporting large write-downs related to bad investments in the U.S. subprime mortgage market, the CEO of Gulf state firm Dubai International Capital LLC said Tuesday, according to a report.

The New York-based diversified financial group has already raised about $30 billion in capital from investors since late last year. Gulf states making investments in Citigroup include Abu Dhabi with $7.5 billion in November. The bank also received $14.5 billion in January from investors including companies run by Singapore and Kuwait. Saudi Prince Alwaleed Bin Talal recently raised his investment in the firm.

Sameer al-Ansari, the chief executive of the Dubai agency said more would be necessary, according to Reuters.

It will take a lot more than that to rescue Citi and other financial institutions, al-Ansari said at a private equity conference in Dubai. The state-owned Dubai firm manages around $13 billion in assets, including investments in HSBC Holdings Plc , and India's ICIC Bank Ltd.

Shares of Citigroup fell 64 cents, or 2.77 percent to $22.45 in pre-market trading.