Microsoft Chief Executive Officer Satya Nadella kicked off one of the largest layoffs in tech history Thursday, signaling he intended to shake up the aging PC industry titan, but leaving questions about how exactly he would transform it into a nimbler, Web-based rival to Apple Inc and Google Inc.

Microsoft Corp said Thursday it will slash up to 18,000 jobs, or 14 percent of its workforce, over the next 12 months as it almost halves the size of its newly acquired Nokia phone business and tries to become a cloud-computing and mobile-friendly software company.

The larger-than-expected cuts are the deepest in the software giant's 39-year history and come five months into Nadella's tenure.