After talks with visiting South African President Jacob Zuma in London, Brown said Zimbabwe must show progress in key areas including democratic reforms.
Zuma, on a three-day state visit to the UK, has suggested sanctions should be eased to help Zimbabwe “move forward”.
The two leaders effectively disagreed over the issue.
After talks in London, the two leaders sought to present a united front on Zimbabwe, with the prime minister praising South Africa’s role in helping bring “stability and change” to its neighbour.
But differences remained and Zuma had made it clear before his visit that if lasting change did not happen in Zimbabwe, some people could use international sanctions as an excuse.
President Robert Mugabe and Prime Minister Morgan Tsvangirai formed a unity government last year but remain deadlocked over outstanding issues of the global political agreement.
The ongoing trial of Roy Bennett, Tsvangirai’s party treasurer, for terrorism and treason has also raised tensions.
The European Union a fortnight ago said sanctions were still needed to keep pressure on Mugabe to live up to his commitments on political and economic reform.
But Zuma has said easing the measures could help the country resolve outstanding political differences.
Brown said some sanctions had been lifted and those remaining in place targeted individuals with a history of supporting violence, not ordinary Zimbabweans.
It was “vital” that commissions set up by the government to increase protection for human rights and freedom of speech and to support democratic institutions concluded their work quickly, he said.
“The UK has always said we are ready to support progress on the ground,” Brown said. “But we must be absolutely sure that progress is being made. We must be moving from what is a unity, transitional government to free and fair elections.”
Zuma said he was “very positive” that progress was being made in Zimbabwe.
He added that the international community now had a “better understanding” of what was happening in the country.
This week, the US announced it would extend sanctions on Zimbabwe for another year, saying its protracted political crisis remained unresolved despite the power-sharing agreement.
Both the EU and the US maintain a travel ban and asset freeze on President Mugabe, his wife and inner circle in protest at disputed elections and alleged human rights abuses by his government.
“I am continuing for one year the national emergency with respect to the actions and policies of certain members of the government of Zimbabwe and other persons to undermine Zimbabwe’s democratic processes or institutions,” US President Barack Obama said in an executive order this week. “These actions and policies continue to pose an unusual and extraordinary threat to the foreign policy of the United States.” — BBC/Staff Writer.