Oil and gas company Saudi Aramco has invited banks to pitch for an advisory position on what is expected to be the world's biggest initial public offering, sources familiar with the matter told Reuters on Tuesday.

Morgan Stanley and HSBC are among banks that have received the request for proposals, one of the sources told Reuters, with the other adding that the invitation was to evaluate Aramco's business and help it with measures surrounding the share sale. The listing of Aramco is a central plank of the government's ambitious plan to transform the kingdom by enticing investment and diversifying the economy away from a reliance on oil.

Aramco's chief executive Amin Nasser said in October that 2018 was being targeted for the flotation of up to 5 percent of the company, though the exact size of the offering will be determined by the Saudi Supreme Council.

The first source told Reuters that appointment of banks was expected before the end of 2017.

Morgan Stanley declined to comment. HSBC declined to comment. Aramco was not immediately available to comment. Speaking in October, Nasser also said Aramco had yet to finalize the location of the listing and was currently reviewing several markets including New York, London, Hong Kong and Japan.

Bloomberg reported in April last year that Aramco had selected JPMorgan Chase & Co and Michael Klein as advisers on the IPO.