"What is important here is actions and not words. We want to see real, serious power-sharing by the Mugabe regime," said State Department spokesman Robert Wood.
"As you can understand, we are a bit skeptical. These types of things have been announced before. The key is always implementation," he added. "The jury is still out."
Opposition leader Morgan Tsvangirai announced his decision to accept a power-sharing arrangement with Mugabe after a meeting of his Movement for Democratic Change’s National Council. He is now set to become prime minister.
The former Bush administration had called for Mugabe’s ouster, saying it was time for the octogenarian leader, blamed for Zimbabwe’s economic ruin, to quit. The country has the world’s highest inflation rate and a worthless currency.
Wood declined to repeat the call for Mugabe’s ouster but said the Zimbabwean leader had abused power and was responsible for the economic and humanitarian crisis in his country.
More than half of Zimbabwe’s population needs food aid and the country is suffering from a cholera epidemic that has killed 3,100 people and infected 60,000.
"What Robert Mugabe needs to do is what is best for the people of Zimbabwe — an effective power-sharing arrangement, one that is equitable, fair and in line with the will of the Zimbabwean people," Wood said.
Mugabe and Tsvangirai signed their power-sharing deal in September but implementation was held up by squabbles over who would get top cabinet positions.
The United States had said previously it would not provide development aid or lift sanctions against Zimbabwe until there was firm evidence of long-term political and economic reform.
The United States and Britain have targeted sanctions against Mugabe and some of his entourage.
Wood said the Obama administration was looking closely at U.S. policy towards Zimbabwe and made clear any change hinged on the kind of government that was in place.
"If and when there is a government in place in Zimbabwe that reflects the will of the Zimbabwean people, the United States will then look to see what we can do to continue to help the Zimbabwean people," Wood said.
He said, however, that Washington would provide assistance to ease the humanitarian crisis.
The U.S. government has provided more than $264 million in humanitarian assistance since October 2007 to help deal with the food and health crisis. This week, the United States paid for 440,000 bars of soap to distribute in areas affected by the cholera outbreak.