Robert Mugabe will remain as president under the proposed unity government
The US says it will only consider easing sanctions against Zimbabwe when it sees evidence of real power-sharing between the rival parties.
A state department spokesman said new aid for Harare was dependent on inclusive and effective governance.
The statement comes a day after a similar announcement by the UK.
Meanwhile, parliament has delayed a debate on changes to the constitution to allow opposition leader Morgan Tsvangirai to become prime minister.
The session was postponed until negotiators from President Robert Mugabe's Zanu-PF party and the opposition Movement for Democratic Change (MDC) return from South Africa, where they have been trying to reach an agreement on details of a power-sharing deal.
The MDC agreed on Friday to join a national unity government with Zanu-PF after months of bitter wrangling.
'True power sharing'
US state department spokesman Robert Wood said "the success or failure of such a government will depend on credible and inclusive power sharing by Robert Mugabe and his Zanu-PF party".
"The US will only consider new development assistance and easing of targeted sanctions when we have seen evidence of true power sharing as well as inclusive and effective governance."
He added: "We will continue to provide humanitarian assistance to the Zimbabwean people in their time of suffering."
Mr Wood also called on the international community to "continue to scrutinise actions by Mr Mugabe to ensure adherence to the letter and spirit of this agreement, including respect for human rights and the rule of law".
The UN says cholera has infected 65,000 people in Zimbabwe
On Tuesday, a British cabinet member said sanctions must be maintained to "keep the squeeze" on Mr Mugabe and his inner circle until they show they have changed course.
Africa Minister Lord Malloch-Brown spoke to the BBC from an African Union summit in Addis Ababa, Ethiopia, where leaders had called for the sanctions to be lifted.
"There is a misunderstanding of what these sanctions are. They are aimed at the individuals - and the companies supporting these individuals - around Mr Mugabe," he said.
"They are not aimed at the country of Zimbabwe or its people."
Donors have said they would only provide aid once a unity government is in place.
Under last week's deal, Mr Tsvangirai will be sworn in as prime minister on 11 February and Mr Mugabe will stay as president.
A power-sharing deal between the MDC and Zanu-PF was signed last September, but got mired in ever more bitter disputes.
The unity government is intended to ease Zimbabwe's economic meltdown but correspondents say this is largely dependent on the restoration of foreign aid and investment.
Zimbabwe is enduring rampant inflation and an escalating food crisis.
Meanwhile an outbreak of cholera, fuelled by the collapse of infrastructure, has now infected nearly 66,000 people and killed more than 3,300.