"[Lonmin] must address the needs of the workers. We are not party to the violence," the union official told reporters in Johannesburg on Friday.
The union was asked why it did not sign a peace accord reached on Thursday after a shooting last month at Lonmin's Marikana mine in the North West.
The Marikana worker representatives also did not sign the agreement.
The parties who did sign the peace accord in the early hours of Thursday morning were Lonmin, and trade unions the National Union of Mineworkers, Solidarity and Uasa.
The accord was aimed at paving the way for wage negotiations and included a commitment to promoting a peaceful work environment.
Amcu president Joseph Mathunjwa said the union had not received another mandate from workers, so it could not approach Lonmin with another wage offer.
"This accord flies in the face of fairness," he said.
He said neither he nor his union ever put a gun to anyone's head, so there was no need to sign a peace accord. – Sapa