Zuma arrived at Harare International Airport around 5:00 pm (1500 GMT) and was met by cabinet ministers and members of President Robert Mugabe's ZANU-PF party. Mugabe was launching a census drive elsewhere.

He made no remarks after landing, but headed directly to his meetings.

The South African president has not visited Zimbabwe since 2010, but after years of stalling, the country last month finished a draft constitution that could send voters to a referendum by the end of the year.

That would be the first balloting since a 2008 presidential run-off that long-ruling Mugabe won -- after his arch-rival Morgan Tsvangirai pulled out of the race in hopes of ending attacks that left more than 200 of his supporters dead.

Under intense regional pressure, they formed a unity government tasked with drafting a new constitution and enshrining greater political freedoms -- with the ultimate goal of new elections.

Zuma now wants an update on the constitution-making process, before heading to a regional summit Friday in Maputo.

"President Zuma is coming to be given an update by the principals on the progress made so far," Mugabe's spokesman George Charamba told AFP.

"President Zuma is not going to negotiate anything or push for anything. His visit comes against a background of tremendous progress made in terms of the constitution-making process and in respect of the consolidation of peace in the country."

Zuma is scheduled to meet separately with Mugabe and Tsvangirai, Charamba said. He will also meet with Welshman Ncube, who heads a faction of Tsvangirai's Movement for Democratic Change and whose son is married to one of Zuma's daughters.

The work of the compromise government has been characterised by frequent haggling over government posts and counter-accusations of violence.

The parties have both threatened to pull out of the power-sharing government.

Early this year, Mugabe threatened to reject Zuma as negotiator if he showed any bias in his mediation.

Zuma will want to shepherd Zimbabwe toward its referendum, which if held later this year could lead to elections in 2013.

The draft constitution would rein in presidential powers while bolstering those of parliament. It would also set down a presidential term limit of 10 years and strip away the president's immunity from prosecution after leaving office.

The MDC has endorsed the draft charter as the best basic law in the country's history, adding that it would be a lost chance if the proposed constitution is rejected.

Mugabe's ZANU-PF says it wants parts of the constitution amended before it is passed -- particularly on issues of who can receive citizenship and how provincial governors are appointed.