UK Foreign Secretary David Miliband said Zimbabwe's new administration would have to make significant progress before the lifting of sanctions was considered.
Meanwhile, a senior US diplomat told the BBC that Washington wanted to help Zimbabwe, but would need to see proof that Mr Mugabe had relinquished some genuine power to Mr Tsvangirai, leader of the Movement for Democratic Change (MDC).
African leaders shake hands following the signing
US Assistant Secretary of State for African Affairs, Jendayi Frazer, said: "We haven't yet had a chance to study the details of the agreement, nor do we know who will be in the cabinet of this new government, so in some ways it's a bit premature for us to comment until we have the full picture."
On Monday, Mr Mugabe, Mr Tsvangirai and Arthur Mutambara - leader of a breakaway MDC faction - shook hands to rapturous applause having signed the agreement in front of some 3,000 invited guests.
Despite the agreement, South African President Thabo Mbeki, who had brokered power-sharing negotiations since July, warned the unity government's full composition was yet to be finalised.
'No instant cure'
The full details and content of the deal have not been confirmed but it proposes a 50-50 division of power, with Mr Mugabe remaining head of state and head of the cabinet.
DEAL'S KEY POINTS
Chairs National Security Council (ministers + security chiefs)
Zanu-PF has 15 ministers
Chairs council of ministers
Runs the country day-to-day
Member of National Security Council
MDC has 16 ministers - 3 from smaller faction
Both men needed to dissolve parliament
End violence, abusive language
Free political activity
Carry out land audit, UK urged to compensate white farmers
Demand end to international sanctions, calls for regime change
Mr Tsvangirai will head a council of ministers, which will be responsible for the day-to-day managing of the country's affairs.
The MDC and its breakaway faction are expected to have 16 ministers, while President Mugabe's Zanu-PF party will have 15.
At Monday's ceremony in Harare's International Conference Centre, Mr Mugabe said there were "lots of things" in the deal that the leaders did not like but that they would work together to "find our way".
Mr Tsvangirai said the agreement was a "product of painful compromises" and that it did not provide "an instant cure" to the fortunes of Zimbabwe.
The new prime minister said addressing the hardships faced by Zimbabwe's people would be his first priority.
"First we will stop the devastating food shortages," he said. "The policies of the past years have made Zimbabwe a nation where the healthy flee and the sickly die."
Mr Mugabe, in power since independence from Britain in 1980, won a controversial presidential run-off election in June.
He ran unopposed after Mr Tsvangirai withdrew, claiming the MDC was the target of state-sponsored violence.
In the first round of the presidential election in March, Mr Tsvangirai gained more votes than Mr Mugabe but official results say he did not pass the 50% threshold for outright victory.
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