"Harassment of citizens and groups needs to stop and reform needs to move forward, so people can cast their votes in elections that are fair and free and credible," Obama said during a visit to South Africa.
Veteran president Robert Mugabe has set elections for July 31, drawing fierce criticism from his political foes.
Obama was speaking at a joint press briefing with President Jacob Zuma at the Union Buildings on Saturday.
Critics accuse Mugabe of attempting to push through a vote before reforms that would clean up the electoral roll, free the media and limit the military's political role.
Prime Minister Morgan Tsvangirai – who will face Mugabe in the polls – has threatened to boycott any unilaterally declared election date.
Tsvangirai won the most ballots in the first round of the 2008 elections, but pulled out of the second round amid violence against his supporters.
Agoa extension
Obama also pledged support for the push to have the African Growth and Opportunity Act (Agoa) extended.
Obama said he supported moves to "improve" and "renew" Agoa.
Agoa – which allows Southern African countries to ship certain products to the US tariff-free – expires in 2015.
Obama said he would first have to convince the US Congress, which had to approve the extension of the Act. The extension of Agoa formed part of bilateral discussion between Obama and Zuma, and their respective delegations.
Zuma also re-affirmed the need for Agoa to be extended.
"Our mutual trade has reached the levels preceding the global recession, largely due to the Act," Zuma said. "Arising out of this visit we would like to see increased investment in the South African economy for mutual benefit."
There were currently 600 US companies operating in South Africa, which have created more than 150 000 jobs. "The US is also a major export market for South African products. South Africa in turn is your biggest market in Africa, accounting for more than $7-billion of exports," Zuma said.
Zuma said South Africa had put several bankable projects on the table. These included projects on infrastructure development and skills development for the youth. "We have urged that underpinning these investments should be the drive for regional integration, industrialisation and localisation of supply and manufacture."
Violence in Egypt
On matters further north of the Africa Obama expressed concern about clashes in Egypt and called on President Mohamed Morsi to be more "constructive" along with the opposition to end the political crisis.
"We are all looking at the situation there with concern," Obama said, adding that the US government had taken steps to ensure the safety of its embassy, consulates and diplomats in Egypt.
"We would urge all parties to make sure they are not engaging in violence and that police and military are showing appropriate restraint," he said. "Everybody has to denounce violence. We would like to see the opposition and President Morsi engage in a more constructive conversation about to move their country forward."
Obama also said that Washington had consistently supported democracy in Egypt, but "it has been challenging given that there is not a tradition of democracy in Egypt".
Egypt was braced for nationwide protests against Morsi on Sunday to mark the anniversary of his turbulent first year in office after violence at rival demonstrations killed three people, including an American.
Morsi (62) stands accused by his critics of failing the 2011 revolution that brought him to power and of ignoring nearly half of the electorate of around 50 million who did not vote for him last year.
Lack of progress in the Middle East
President Zuma on Saturday said that South Africa remained concerned at the lack of progress in the Middle East peace process.
"We unequivocally support the Palestinian bid for statehood and believe in the principle of a two-state solution," said Zuma, adding that South Africa had noted the US's latest attempts to revive negotiations.
"At the same time we are of the view that lasting peace in the Middle East will not be possible without addressing the other ongoing conflicts in the region, which are a source of much insecurity and instability."