Dutch banking giant ABN Amro has rejected a $24.5 billion (€17.99 billion) offer for its US bank LaSalle from a consortium led by Ulster Bank-owner Royal Bank of Scotland (RBS), but said it would let shareholders express their views on the institution's next steps.

ABN maintained the offer by RBS and its partners Santander and Fortis was not superior to Bank of America's deal to buy LaSalle for $21 billion because it was conditional on them acquiring all of the ABN group and carried execution risks.

A deal to buy out ABN would be the world's largest bank takeover and is seen as the first step in wider consolidation among Europe's banks.

The RBS consortium has proposed a €72.35 billion offer at €39 per share, including a €0.60 dividend, for all of ABN, but the Netherlands' biggest bank said the consortium's financing, as well as various regulatory, tax and legal issues carried "uncertainty and execution risks".

Sources familiar with the weekend talks said the main sticking point was that the offers for LaSalle and ABN were conditional on each other.

ABN's lawyers advised the bank that the interconditionality of the offers could breach the terms under which the Bank of America deal could be undone, one source said.

RBS, Belgian-Dutch Fortis and Spain's Santander said in a statement that they considered their offer for LaSalle to be a superior proposal as it was "materially greater" than the Bank of America bid. The RBS-led group is trying to buy ABN and beat out an $88 billion offer by Barclays for the Dutch bank, announced two weeks ago, along with the related deal to sell Chicago-based LaSalle to Bank of America.

Shareholders revolted against the Barclays and Bank of America deals, and succeeded last week in getting a court in Amsterdam to freeze the LaSalle sale. ABN said it would let shareholders decide.

"It is ABN Amro's intention to hold an extraordinary general meeting, the details of which will be made available in due course, to enable shareholders to express their views on the alternatives available to them at that time."

Dutch shareholder group Veb, which sued in a commercial court to stop the LaSalle sale, said it would probably vote against selling LaSalle to Bank of America if put to shareholders.

Shares in ABN fell 2.3 per cent in Amsterdam to close at €35.86. Fortis and Santander shares were flat, while RBS was untraded due to a market holiday in London.