Rumors of a hostile approach from Citigroup come at a time when ABN Amro is being wooed by British bank Barclay's, which had been granted an exclusive right to negotiate terms.
However, once Barclays table a bid for ABN Amro, other banks will be able to mount counter bids, whereupon Citigroup would be expected to be able to make a higher offer.
The possibility of a bid from Citigroup has been lent credence by market analysts who perceive that a takeover by Citigroup would prove more beneficial for both ABN Amro and the US firm.
It is also thought that acquisition of ABN Amro would fit in with Citigroup chief executive Charles Prince's stated aim of boosting the proportion of its revenue from overseas interests from 44 to 60 per cent.
On Wednesday, Barclays and ABN Amro were reported to have come to a preliminary agreement to move the head office of the mooted new company to Holland.