MILAN/ABU DHABI, Jan 11 (Reuters) - Abu Dhabi's Aabar Investments is set to buy into an upcoming 13 billion euro ($14 billion) share issue at UniCredit CRDI.MI to keep its 5 percent stake in Italy's biggest bank unchanged, three sources familiar with the matter said on Wednesday.

UniCredit is expected to launch its jumbo share sale next month as it strives to bolster its capital under new Chief Executive Jean Pierre Mustier.

One of the sources said Aabar had confidence in the bank's future strategy.

To keep its stake at the current level, Aabar would need to invest around 650 million euros.

UniCredit and Aabar had no comment.

($1 = 0.9505 euros)

(Reporting by Paola Arosio and Stanley Carvalho) ((; +39 02 6612 9526;))