American tech billionaire Elon Musk said Monday that his company, SpaceX, could build a self-expanding city on the Moon in under a decade.

According to Musk, his aerospace company also aims to begin building a city on Mars within about five to seven years, although to date no human has set foot on Mars, and no manned Mars mission has been scheduled.

However, the company’s overriding priority is "securing the future of civilization” on the Moon, he said.

"It is only possible to travel to Mars when the planets align every 26 months (six month trip time), whereas we can launch to the Moon every 10 days (2 day trip time),” he wrote on his social media platform X.

"This means we can iterate much faster to complete a Moon city than a Mars city,” he said, adding that building a self-growing city on Mars would take more than 20 years.

Musk has been noted for making overly optimistic predictions about technology. In 2018, for instance, he said autonomous cars made by his company Tesla "could probably do a coast-to-coast drive in 3 months, 6 months in the outside.” And in 2017, he said that SpaceX would be flying private citizens around the Moon the following year.