The Bush administration said Monday it will make a new diplomatic push in the first days of the new year to step up sanctions aimed at forcing Zimbabwean President Robert Mugabe from power. The United States has formally dropped its support for power-sharing between Mr. Mugabe and the political opposition in Harare.

The new harder U.S. tone was set by Secretary of State Condoleezza Rice, who told the French news agency AFP the situation in Zimbabwe is "unconscionable" and that the United States will consult allies in Europe and Africa on tougher sanctions against Mr. Mugabe and his ruling circle.

Rice, who held inconclusive talks on Zimbabwe at the U.N. Security Council in New York last week, questioned how long remaining friends of the Mugabe government can go on calling the crisis in Zimbabwe an internal matter. She said it is high time for the world community to step up sanctions.

U.S. officials have faulted regional power South Africa for failing to use its leverage to bring about change in neighboring Zimbabwe and have accused leaders there of being protective of Mr. Mugabe.

At a news briefing, State Department Spokesman Sean McCormack noted that the composition of the Security Council changes on January 1, a reference to the expiration of South Africa's two-year term as an elected council member.

He said the United States will discuss the broader imposition of targeted sanctions because there is a limit to what those enacted only by western powers can achieve. At the same time he downplayed the idea of trying to close Zimbabwe's borders to force a change in government, saying such action could have unintended consequences:

"I'm not sure that closing a border is necessarily the right thing to do if you have people that are fleeing a disastrous situation, or are fleeing for their lives for fear of political oppression. This is a political question. The fact of the matter is that Robert Mugabe and those who are around him are still there in part because they still have political support, or at the very least countries not come out and spoken out against Robert Mugabe and those around them," he said.

The Bush administration, until two weeks ago, supported South African-mediated power-sharing talks between Mr. Mugabe and the opposition Movement for Democratic Change, the MDC, which won parliamentary elections last March.

But President Bush joined other world leaders December 9 in flatly calling for Mr. Mugabe's departure from power after he refused to yield any major cabinet seats to the MDC.

Secretary Rice sent Assistant Secretary of State for African Affairs Jendayi Frazer to South Africa to articulate the policy change. Frazer told reporters there Sunday a power-sharing arrangement under which Mr. Mugabe remains president is not credible.

She said fresh elections in Zimbabwe are necessary but not possible in the current environment, with a near-total economic breakdown and cholera epidemic. She warned the South African government will be held responsible for what is happening in Zimbabwe because it has more influence with its neighbor that any other country.