LONDON, May 5 (Reuters) - Saudi Aramco has appointed advisory firm Brunswick to join FTI Consulting in running media and investor relations for what is set to be the world's largest initial public offering (IPO), sources said.

The decision to split the role underscores the mammoth task facing Saudi Arabia's national oil giant which hopes to list in 2018 in several countries around 5 percent of the company in a share sale which could raise $100 billion.

Brunswick was recently appointed to run the external and media communications for the IPO while U.S.-based FTI will focus on managing investor relations, according to two sources with knowledge of the matter.

A spokeswoman for Brunswick declined to comment, FTI did not respond to a request for comment. Aramco declined to comment.

FTI's appointment was reported by Reuters in March.

Aramco will list its shares on the Kingdom's stock exchange as well as one or more foreign stock exchanges, Deputy Crown Prince Mohammed bin Salman said earlier this week.

London, New York and Hong Kong have been named as possible exchanges for the listing.

Aramco is courting governments, including China and large institutional investors to buy into the IPO although the full details of the listing, including its size and value, are still unknown.

The $100 billion IPO price tag is based on Aramco being valued at $2 trillion, although some analysts believe the ultimate valuation could be much lower.

(Reporting by Ron Bousso; Additional reporting by Rania El Gamal; Editing by Elaine Hardcastle) ((ron.bousso@thomsonreuters.com; +44)(0)(2075422161; Reuters Messaging: ron.bousso.reuters.com@reuters.net))