UNITED NATIONS — With the support of the Group of Eight industrialized nations, America will move as early as tomorrow to impose U.N. Security Council-backed sanctions on the regime of Robert Mugabe, seeking to increase the pressure on the aging Zimbabwean leader and further delegitimize his hold on power.

Meeting in Japan yesterday, leaders of the G-8 issued a statement calling for “financial and other measures against those individuals responsible for the violence” in Zimbabwe.

Mr. Mugabe, 84, lost a presidential election in March to opposition leader Morgan Tsvangirai. But after forcing a second round of voting in which he eventually ran unopposed, Mr. Mugabe won a June runoff that President Bush and other heads of state have criticized as a “sham.”

The G-8 statement will likely increase tensions between the world’s top industrial nations, whose leaders are meeting for an annual summit, and African leaders loyal to South Africa’s President Mbeki, who has emerged as Mr. Mugabe’s strongest ally. African leaders have lobbied the G-8 against imposing sanctions, and South Africa, which holds a temporary seat on the Security Council, is expected to lead an uphill fight there against the American-led resolution.

Although President Medvedev of Russia signed on to the G-8 statement, his U.N. ambassador, Vitaly Churkin, expressed reservations yesterday about council-backed sanctions. Mr. Churkin called on council members to “listen” to South Africa’s argument and said sanctions could be harmful for the ongoing diplomatic “mediation.” That effort, led by Mr. Mbeki, is aimed at preserving Mr. Mugabe’s hold on power while giving members of Mr. Tsvangirai’s party a shared role in government.

European and Latin American countries on the 15-member council are expected to support the sanctions resolution, as is at least one African representative, Burkina Faso. The proposed sanctions include a weapons embargo and a travel ban for Mr. Mugabe and several members of his inner circle, as well as a freeze on their financial assets. While Russia and China will not support the resolution, diplomats here say they doubt they will veto it.

Yesterday’s G-8 statement was signed under intense lobbying by Mr. Bush and Prime Minister Brown of Britain. In addition to sanctions against Zimbabwe, it called on the United Nations to appoint a new envoy to help negotiate a resolution to the crisis in the country.

Several African candidates whom diplomats named as candidates — including a former U.N. secretary-general, Kofi Annan, and a former Nigerian president, Olusegun Obasanjo — have a much higher profile than Secretary-General Ban’s current mediator, Haile Menkerios, a mid-level U.N. bureaucrat.

Appointing a new envoy would undoubtedly be seen as a no-confidence vote in Mr. Mbeki’s current mediation.

A deputy U.N. secretary-general, Asha-Rose Migiro, said yesterday that in her recent consultations with African leaders, “they supported the continued role of Mr. Menkerios.”

Ms. Migiro, a former Tanzanian foreign minister recommended for her post by South Africa, expressed support for Mr. Mbeki’s mediation.

The South African ambassador to the United Nations, Dumisani Kumalo, said his country has “a problem” with any mediation proposed by the Europeans, who he said have a “hidden agenda” and who have expressed public support for Mr. Tsvangirai.

Mr. Churkin said some of the sanctions America has proposed are “quite excessive,” and said the G-8 statement did not refer the decision to impose sanctions to the Security Council deliberately.

“We believe that the G-8 has provided the support needed for us here to move,” the American ambassador to the United Nations, Zalmay Khalilzad, said. “Absent a veto, which we do not anticipate,” but cannot rule out, “the votes are there to move forward on this resolution.”