Elon Musk has suggested he is “doing [his] bit” to address “underpopulation” after it emerged he had fathered twins with one of his employees.

The billionaire entrepreneur appeared to make oblique references to his latest offspring on Twitter on Thursday, after news broke that Shivon Zilis, a top executive at Mr Musk’s neurotechnology firm Neuralink, had birthed twins fathered by him in November 2021.

“A collapsing birth rate is the biggest danger civilization faces by far,” Mr Musk, now a father of nine, wrote on Twitter.

“Doing my best to help the underpopulation crisis,” he said in another tweet.

The twins’ birth was first reported by news website Business Insider, citing US court documents where Mr Musk and Ms Zilis had filed a petition to change their names.

The parents want their children to "have their father's last name" and for their middle names to contain their mother's last name, said the legal filings. A month later, a Texas judge approved the petition, the report added.

Little is known about the latest twins so far, with US media reporting that their names are known to journalists but have been withheld to preserve their privacy.

Ms Zilis, 36, is identified on her LinkedIn profile as director of operations and special projects at Neuralink, which was co-founded and chaired by 51-year-old Mr Musk.

Reports claim Ms Zilis is one of the people Mr Musk could install to run Twitter after his $44bn acquisition of the social media website completes.

She started working at Neuralink in May 2017, the same month she was named a project director in artificial intelligence at Tesla, where she worked until 2019.

Ms Zilis also serves as a board member at artificial intelligence research firm OpenAI, which was co-founded by Mr Musk, according to her profile on LinkedIn.

News of the twins' arrival brings Mr Musk's total offspring count to nine. The billionaire shares two children with Canadian singer Grimes. Mr Musk said he and Grimes were "semi-separated", according to reports in September last year, while 34-year-old Grimes told Vanity Fair magazine in March: “I would probably refer to him as my boyfriend, but we’re very fluid.”

The oldest Musk child by Grimes is a boy named X Æ A-12 (pronounced “X AI Archangel”), who is now two years old, while their sister Exa Dark Sideræl Musk (also known as “Y”) was born in December to a surrogate mother.

Musk’s first child by his first wife Justine Wilson died in 2002 at the age of 10 weeks from sudden infant death syndrome. Ms Wilson bore him five more children, all boys and all consisting of twins and triplets.

The oldest of the Musk brood, the twins, are now in their teens and at least one wants nothing to do with their father.

18-year-old Xavier Alexander Musk made a legal decision to publicly disown Mr Musk earlier this year. Xavier filed legal papers in April seeking to change his name to Vivian Jenna Wilson and to change his legally-recognised gender.

In a widely-reported legal filing the Musk child wrote that his reasons for seeking to transition were: “Gender identity and the fact that I no longer live with or wish to be related to my biological father in any way, shape or form.”

Xavier has a twin brother, Griffin Musk, who has not sought public attention.

Three triplet brothers were born in 2006.

Neuralink, the company for which Ms Zilis works, specialises in making brain interface technology. While it has yet to start trials on humans, it has reportedly conducted experiments where probes were inserted into rats’ heads and machines were able to decode electrical signals from within the mammals’ brains.

Mr Musk has long puzzled over US fertility rates, having said last year that “civilization is going to crumble” unless more children are born in America.

In another tweet on Thursday, Mr Musk wrote: “Population of Mars is still zero people!”

His company SpaceX has long been linked to a potential private venture mission to Mars, having previously advertised for people willing to undertake a one-way journey to the Red Planet.