The battle for control of Arconic (ARNC) - Get Arconic, Inc. Report heated up Monday as the company's independent directors published a letter to shareholders defending the tenure of under-pressure chairman and CEO Klaus Kleinfeld.

The directors in the letter highlighted Kleinfeld's role while CEO of Alcoa (AA) - Get Alcoa Corp. Report , cobbling together the assets that became Arconic, and then spinning the unit off as an independent. Kleinfeld came to Alcoa in 2008, during a prolonged slump in aluminum prices, and devised a strategy to lessen the company's exposure to commodity prices and ease its long-term debt burden to allow it to better compete against international rivals.

The directors noted that pre-split Alcoa holders have seen their investment increase by a combined 32%, including dividends from Nov. 1, 2016, to Jan. 27, 2017, while Arconic's stock price has increased by 19.5% during this period. And though Arconic has stumbled in recent quarters, due at least in part to program delays experienced by key customers, they said the company is focused on improving operations.

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"Mr. Kleinfeld and the management team are 100% focused on continuing to improve operating results, expand margins and improve return on net assets," the directors wrote. "In connection with the creation of Arconic as a newly standalone company and as part of its active oversight role, the board will create a finance committee that will be focused on maximizing return on investment and capital efficiency across the company."

The letter comes just days after activist Paul Singer's Elliott Management, owner of more than 10% of Arconic shares, formally nominated a slate of five directors to the company's board and suggested former Spirit AeroSystems (SPR) - Get Spirit AeroSystems Holdings, Inc. Class A Report CEO Larry Lawson as Kleinfeld's replacement. Expect the rhetoric to continue to build as the two sides grow closer to the company's yet-to-be-scheduled annual meeting.

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Elliott has complained that Arconic has underachieved under Kleinfeld, in a letter to shareholders last week highlighting that revenue and earnings in recent years have repeatedly come in below initial guidance. They say some of the acquisitions that built Arconic, notably Alcoa's $3 billion deal for Firth Rixson, have not yet justified the substantial premiums paid, and are pushing to see margins improved.

Elliott already has a foothold at Arconic, the result of a February 2016 settlement with Alcoa that led to three dissident directors appointed to the board. Those three directors, Ulrich Schmidt, a former CFO of Spirit AeroSystems, John Plant, ex-CEO of TRW Automotive and Sean Mahoney, a former investment banker, were among the signatories to Monday's letter supporting Kleinfeld, though there is a strong possibility that an Elliott victory in a proxy battle would send a message to those three directors to shift their backing to the activist fund.

A key consideration is that Kleinfeld's director position is being contested this year, even though only a segment of the board is up for election at the annual meeting. A shareholder vote removing Kleinfeld from the board, combined with the election of Elliott's new nominees and the directors that they had previously recommended, could be enough to take over the board and push Kleinfeld out of the CEO position.

The established directors on Monday said they are confident that better times are ahead, saying "we know there is more the company can do and that there is even greater value potential to be unlocked, and we are driving the company and management in pursuing stronger performance and maximizing shareholder value at Arconic."

They also said Kleinfeld is the right person to do the job.

"We are confident that we have the right strategy and the right team, and that the company is in the best position it has enjoyed since the financial crisis," the directors wrote.

- Ron Orol contributed to this article