The Reagan administration said today it "cannot accept" a statement from jailed black nationalist leader Nelson Mandela that violence is the only option left in South Africa.

Mandela was quoted in today's edition of The Washington Times as saying that there is "no alternative" to violence and "no room for peaceful struggle" in the effort by blacks against apartheid.

"We cannot accept his formula that violent change is the only option left in South Africa," said presidential spokesman Larry Speakes. "There is and must remain an alternative to violence. We call for peaceful, rapid change away from apartheid.

"As far as Mandela is concerned, he is an important South African black leader and a key figure in South Africa's political scene. We have called for his release," he added.

Mandela has been imprisoned since 1964. South African President Pieter W. Botha last week said again that he would not release Mandela unconditionally; Botha has demanded that Mandela first renounce violence.

Speakes said the issue of conditions for Mandela's release is up to the South African government. "We want him released," he said, but added that the United States does not condone violence.

Although the United States is seeking "prompt" negotiations on South Africa's future, Speakes said the administration would "not define" who should sit at the conference table, a matter he said is "up to the South Africans."

Also today, Speakes said President Reagan has not made a decision on how to handle legislation pending in Congress that would impose economic sanctions against Pretoria. He was responding to a report that Reagan intends to veto the legislation but may impose some provisions by executive order.

So far, Speakes said, "nothing has come to him."