Billionaire Carl Icahn has cut his stake in Occidental Petroleum, the activist investor said Friday in a letter to shareholders of the US oil major.

Icahn said his investment, which he has reduced by 10 million shares to almost 23 million shares valued at almost $900 million, has become risky "and without changing the incumbent Board and potentially the CEO, and in the absence of accountability for the OxyDarko Disaster, I am very concerned."

The "OxyDarko Disaster" Icahn refers to is the $38 billion (or $57 billion with assumed debt) takeover of Anadarko Petroleum which bypassed an Occidental shareholder vote.