BOSTON (Reuters) –- Activist investor Carl Icahn stepped up his battle against Steel Partners on Friday when he urged other investors to join him in opposing the hedge fund firm's recently announced plans to go public.

Icahn sent a four-paragraph-long letter to investors on Friday, seeking to rally support and bring new public attention to a private fight that has simmered for weeks.
The billionaire investor regularly takes large stakes in companies and then agitates for change.
Last week Icahn took his first public step when ACF Industries, the world's leading railcar and railcar components maker which is heavily affiliated with him, sued Steel Partners in Delaware Chancery Court charging that the hedge fund firm had committed fraud.
The lawsuit said Steel Partners was trying to turn its main fund into a publicly traded company instead of selling assets to return the money that investors had demanded to get back late last year when the fund suffered heavy losses. At the time, Steel Partners said it felt the suit had no merit and vowed to contest it vigorously in court.
Before that, Icahn and Steel Partners' Warren Lichtenstein had spoken by telephone late last year. According to the court documents, Lichtenstein had assured Icahn that he was working on a plan to satisfy investors' redemption requests, that investors would be informed and that no action would be taken to implement a plan until February at the earliest.
But in a letter sent by Lichtenstein on New Year's Eve, all had changed, the court papers said, alleging the fund firm pulled a ""bait and switch.""
Steel Partners did not return an email or call seeking comment on Friday after Icahn published his letter.