WASHINGTON - The United States is seeking "a new era" for Zimbabwe, the State Department's top official for Africa said on Thursday, implicitly calling on long-time President Robert Mugabe to step aside as a political crisis mounts.In an interview with Reuters, acting Assistant Secretary of State for African Affairs Donald Yamamoto appeared to dismiss the idea of Mugabe, who has ruled Zimbabwe for 37 years, remaining in a transitional or ceremonial role."It's a transition to a new era for Zimbabwe, that's really what we're hoping for," Yamamoto said.Zimbabwe's army seized power this week, in an apparent effort to prevent Mugabe, 93, from handing power to his wife. He has not resigned or been formally deposed, however, and he was pictured on Thursday shaking hands with the military chief, Constantino Chiwenga.Yamamoto, speaking on the sidelines of a meeting with African Union officials at the State Department in Washington, described the situation in Zimbabwe as "very fluid."The United States would discuss lifting multiple US sanctions on Zimbabwe if it began enacting political and economic reforms, he said.In a message for Zimbabwe's political leaders, he said: "Our position has always been that if they engage in the constitutional reforms, economic and political reforms, and move forward to protecting political space and the human rights, then we can start the dialog on lifting sanctions."The United States has not given aid to Zimbabwe's government for many years, but provides development aid to nongovernmental groups, particularly for healthcare."Now whether we give to the government, that depends on what happens in Zimbabwe," Yamamoto said.US Secretary of State Rex Tillerson is due to meet on Friday in Washington with 37 African foreign ministers.