Tech billionaire Elon Musk has said that he prevented a Ukrainian attack on a Russian Navy base last year by declining Kyiv's request to activate internet access in the Black Sea near Moscow-annexed Crimea.

Satellite internet service Starlink, operated by Musk-owned company SpaceX, has been deployed in Ukraine since shortly after it was invaded by Russia in February 2022.

"There was an emergency request from government authorities to activate Starlink all the way to Sevastopol. The obvious intent being to sink most of the Russian fleet at anchor," Musk posted Thursday on X, formerly named Twitter.

The city of Sevastopol is the base of Russia's Black Sea Fleet on the Crimean peninsula, which was annexed by Moscow in 2014.

"If I had agreed to their request, then SpaceX would be explicitly complicit in a major act of war and conflict escalation," Musk said.

Musk was posting in response to a published excerpt of an upcoming biography of the tech tycoon by Walter Isaacson.

In the excerpt published by The Washington Post on Thursday, Isaacson wrote that in September last year, "The Ukrainian military was attempting a sneak attack on the Russian naval fleet based at Sevastopol in Crimea by sending six small drone submarines packed with explosives, and it was using Starlink to guide them to the target."

Musk had "spoken to the Russian ambassador to the United States... (who) had explicitly told him that a Ukrainian attack on Crimea would lead to a nuclear response," Isaacson wrote.

Musk "secretly told his engineers to turn off coverage within 100 kilometers of the Crimean coast. As a result, when the Ukrainian drone subs got near the Russian fleet in Sevastopol, they lost connectivity and washed ashore harmlessly", according to Isaacson.

In another post on Thursday, Musk countered Isaacson's account.

"The Starlink regions in question were not activated. SpaceX did not deactivate anything," Musk posted.

Russia's ex-president and senior security official Dmitry Medvedev, in response to Isaacson's detailing of the incident, lauded Musk.

"(Musk) was concerned about a retaliatory nuclear strike," Medvedev posted on X Thursday.

"If what Isaacson has written in his book is true, then it looks like Musk is the last adequate mind in North America. Or, at the very least, in gender-neutral America, he is the one with the balls."

Musk also called Thursday for a truce in the conflict.

"Both sides should agree to a truce. Every day that passes, more Ukrainian and Russian youth die to gain and lose small pieces of land, with borders barely changing. This is not worth their lives," he posted.