Microsoft has narrowed its list of external candidates to replace chief executive Steve Ballmer to about five people, including Ford CEO Alan Mulally and former Nokia CEO Stephen Elop, "sources familiar with the matter" have told the Reuters news agency.

Three internal candidates are also on the short list, the sources said.

Despite the narrower list - the company started with about 40 names - the process could take a few more months, the sources told Reuters. In August, Ballmer said he would retire within 12 months.

The names of other candidates could not be learned but the search committee is interviewing executives from a wide range of sectors, including life sciences and consumer, the sources said.

Microsoft declined to comment to Reuters on the process and on behalf of the internal candidates.

Ford spokesman Jay Cooney told the news agency: "There is no change from what we announced last November. [Mulally] remains fully focused on continuing to make progress on our One Ford plan. We do not engage in speculation."

Reuters noted that investors have pushed Microsoft's board in recent months to look for a turnaround expert, such as Mulally or Computer Sciences Corp CEO Mike Lawrie, to succeed Ballmer.

Former Boeing executive Mulally, 68, is credited with inspiring a cultural change that helped Ford reverse its losses and avert a federal bailout in 2009.

Reuters said it remained unclear when Mulally would leave Ford, which he has headed since 2006. He has repeatedly said he plans to adhere to his agreement to stay with Ford until the end of 2014.

But people familiar with the matter have previously told the news agency that Mulally is open to other opportunities and may leave Ford earlier if another job presents itself.

Ford on 5 November announced that its design, manufacturing and labour chiefs would all retire on 1 January, 2014.

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