(Adds share price, Ternium appeal)
RIO DE JANEIRO, April 14 (Reuters) - Common shares in Brazil's Usiminas jumped over 16 percent on Wednesday after securities industry regulator CVM demanded its two top shareholders extend a tender offer for the rest of the steelmaker's common shares.
The decision is the latest twist in a struggle for control between the shareholders Luxembourg-based Ternium SA and Japan's Nippon Steel & Sumitomo Metal Corp.
Ternium said on Wednesday it would appeal the CVM decision, while Nippon said it needed to fully understand the details before making a decision.
The two control Usinas Siderúrgicas de Minas Gerais SA, as the company is formally known, through a shareholder accord but have been at loggerheads for six months over the departure of the steelmaker's former chief executive officer, leading to a number of court cases and appeals to the regulator.
The CVM decision relates to last October, when Ternium increased its stake by buying 616.7 million reais ($200 million) worth of shares from the workers' pension fund of state-run Banco do Brasil SA, known as Previ. At the time, Ternium stated the deal would not trigger a tender offer to other minority shareholders.
The decision by the CVM, which was asked to review the deal by Nippon Steel, overturns that and says the offer must be extended by both Ternium and Nippon Steel, according to the Usiminas statement late Tuesday.
"We have received such a notice from CVM," a Nippon Steel spokeswoman in Tokyo said on Wednesday. "We will check on details and consider what measures we would take."
It is unclear how the regulator's ruling would affect the battle for control, Citigroup analyst Alex Hacking wrote in a note to clients.
Ternium offered 12 reais per share in its October offer, an 82 percent premium to the closing price on the day before the deal was announced, but since then the shares have rallied, rising to 19.60 reais on Wednesday.
According to Thomson Reuters data, Ternium owns 26.9 percent of Usiminas while Nippon Steel controls a 29.1 percent stake. Rival steelmaker Companhia Siderurgica Nacional SA owns about 14 percent of Usiminas' common shares.
Ternium and Nippon Steel have been at odds since former Usiminas CEO Julian Eguren, who previously worked at Ternium, was dismissed over allegations of misuse of funds in September. Eguren denies wrongdoing and Ternium has demanded his reinstatement while Nippon Steel has refused.
$1 = 3.06 Brazilian reais Reporting by Stephen Eisenhammer and Alberto Alerigi in RIO DE JANEIRO, Yuka Obayashi in TOKYO; Editing by Lisa Shumaker, Edwina Gibbs and Meredith Mazzilli