Occidental Petroleum (OXY) - Get Report shares jumped higher Friday after billionaire activist investor Carl Icahn trimmed his stake in the oil group while vowing to run a proxy fight against the board next year.

Icahn, who owns around 23 million shares in Occidental valued at around $900 million, was intensely critical of the group's recent $38 billion takeover of Anadarko Petroleum (APC) - Get Report  a deal he called "one of the worst I've ever seen", and one that has cost Occidental more than $21 billion in market value. The new stake is around 10 million less, in share terms, than his earlier holdings, which peaked at around $1.6 billion in late May.

"In our view, the OXY/Anadarko merger made no sense for stockholders, but perhaps it made sense for Vicki Hollub, OXY's President and CEO, and certain Board members who, we believe, were concerned that OXY would be a takeover target, and therefore grossly overpaid to acquire Anadarko in order to protect themselves and their jobs," Icahn wrote in an open letter to shareholders.

"The Company's actions are especially reprehensible since management repeatedly pledged that it would not engage in risky M&A and would maintain a prudent balance sheet," he added.

Occidental shares were marked 2.56% higher by mid-day trading Friday and changing hands at $40.04 each following publication of the Icahn letter, a move that still leaves the stock nursing a year-to-date decline of around 34%.

Earlier this week, Occidental told investors it will slash its capital spending by around 40%, pegging 2020 outlays at around $5.4 billion and those in 2021 at $6.6 billion as it moves towards trimming debt linked to the Anadarko acquisition. 

"We are very focused, we are very intense on ensuring that we get the asset sales done because we believe we must get our debt down, that's an internal target," CEO Hollub told investors on a conference call Tuesday. "We talk about it every day." 

Ichan had been pressing for changes to the Occidental board, and filed court papers in Delware Monday seeking to determine "whether the actions of the directors and management were just serious mistakes or whether they represented knowing intentional breaches of fiduciary duty" in executing the Anadarko deal.

"In my opinion, OXY's acquisition of Anadarko is nothing more than a massive $57 billion levered bet on the price of oil - and the bet is failing," Ichan wrote Friday. "Interestingly, although OXY has dropped 42% since April, oil has only dropped 12%."

"We believe this is because Wall Street has completely lost faith in Hollub and her Board and has concluded, in my view, that Hollub and her Board will put their interests far above the best interests of OXY's stockholders," he added. "Once the institutional investors have soured on a company, it is extremely hard to regain their confidence."