Ms Frazer said she was not optimistic after talks remained deadlocked between Mr Mugabe and his main rival Morgan Tsvangirai.
"If it doesn’t work then we are going to continue the pressure that we’ve put on the Government. We will look at new sanctions against President Mugabe and his regime," Ms Frazer told a group of reporters in Tokyo.
"Right now we’re not so optimistic. It doesn’t look very good for power-sharing," she said.
European Union foreign ministers at a meeting on Monday in Luxembourg also warned of fresh sanctions unless Mr Mugabe respected the power-sharing deal.
Former South African leader Thabo Mbeki was brokering talks to salvage the month-old pact, under which 84-year-old Mr Mugabe would remain President and his rival would take the new post of prime minister.
Mr Tsvangirai has threatened to pull out of the agreement after Mr Mugabe last weekend announced he would award key ministries to his own party.
"The actions of President Mugabe are not consistent with any notion of power-sharing," Mr Frazer said.
"We will have to see whether former president Mbeki will be able to get President Mugabe to agree to what he had agreed to, which was to truly share power," she said.
Ms Frazer, the assistant secretary of state for African affairs, is on a visit to Japan and China for talks on African hotspots and foreign aid cooperation.- AFP