RIYADH  - Saudi Arabia's biggest lender, National Commercial Bank, is set to pick JPMorgan to advise on its merger talks with smaller rival Riyad Bank, several sources familiar with the matter said.

NCB, the kingdom's biggest lender by assets, and Riyad Bank said in December they have begun preliminary discussions to merge, in a move that would further extend NCB's lead over its closest rivals by boosting its assets by almost a third to $183 billion). 

NCB was not immediately available to comment, while JPMorgan declined to comment. Reuters reported on Monday that Riyad Bank has chosen Goldman Sachs for an advisory role.

JPMorgan's selection is subject to ratification by NCB's board, the sources said.

(Additional reporting by Davide Barbuscia; Tom Arnold and Saeed Azhar; Editing by Alexander Smith) ((Saeed.Azhar@thomsonreuters.com; +971 44536787; Reuters Messaging: saeed.azhar.reuters.com@reuters.net))