Apple's longtime artificial intelligence chief is leaving as the iPhone maker and its Siri voice assistant scramble to play catchup with AI rivals, the company said.

John Giannandrea, who had served as Apple's senior vice president of machine learning and AI strategy since 2018, has stepped down -- months after a long-planned AI overhaul of the Siri voice assistant got delayed by bugs and glitches - a situation one executive reportedly called "ugly."

Amar Subramanya, a veteran of Google and Microsoft, will take over as Apple's vice president of artificial intelligence and oversee the teams developing Apple's in-house AI models, machine learning research and AI infrastructure. He will report to Apple's software boss Craig Federighi.

"AI has long been central to Apple's strategy, and we are pleased to welcome Amar to Craig's leadership team and to bring his extraordinary AI expertise to Apple," CEO Tim Cook said in a statement.

Cook added that Federighi "has been instrumental in driving our AI efforts, including overseeing our work to bring a more personalized Siri to users next year."

Giannandrea will serve as an adviser before leaving the company for good in the spring, Apple said late Monday. Other staffers who previously reported to Giannandrea will now report to Apple's chief operating officer Sabih Khan and services head Eddy Cue.

Apple has lagged far behind its Big Tech rivals in developing and integrating AI onto its iPhones and other devices.

When the Siri delay was announced, Apple reassigned oversight of the voice assistant and AI robotics to other executives, Bloomberg reported. Giannandrea had previously overseen those teams.

Wedbush analyst Dan Ives said the shakeup came at the "right time" for Apple given the pressure it faces to right the ship on AI development.

"We believe that Subramanya represents the right hire at the right time with the clock ticking on Apple's AI strategy heading into next year with outside hires a necessary move to improve the AI strategy," Ives said in a note to clients.

In July, Cook said Apple was "significantly growing our investments" in AI and would be "open to M&A that accelerates our roadmap" as it looks to catch up with Big Tech rivals.

Meanwhile, the updated Siri is now expected to launch by this spring.

Apple shares rose about 1% in Tuesday trading.

The leadership shakeup followed reports that Apple has been stepping up its succession planning to identify potential replacements for Cook.

Apple shakes up AI team as top exec John Giannandrea steps down