One of Col Gaddafi's advisers has told the BBC there is no prospect of him stepping down, as the rebels demand.
On Sunday, Mr Zuma's governing African National Congress condemned Nato's air and missile strikes in Libya.
"We... join the continent and all peace-loving people of the world in condemning the continuing aerial bombardments of Libya by Western forces," the party said after a two-day meeting of its executive council.
Nato imposed a no-fly zone in Libya and began bombing Col Gaddafi's forces in March as they threatened to overrun rebel-held parts of the country, a month after nationwide anti-government protests began.
International pressure on Col Gaddafi continues to grow, with the G8 calling for his departure on Friday and Russian President Dmitry Medvedev saying on Saturday he no longer had the right to lead Libya.
The chairman of the Benghazi-based rebel Transitional National Council (TNC), Mustafa Abdul Jalil, welcomed the statements, saying: "The entire world has reached a consensus that Col Gaddafi and his regime have not only lost their legitimacy but also their credibility."
But the Libyan government said it was not concerned by the G8's decisions, saying it was merely an economic summit.
"We are an African country. Any initiative outside the AU framework will be rejected," Deputy Foreign Minister Khalid Kaim said.
Attack helicopters Mr Zuma walked down the red carpet at Tripoli's airport on Monday afternoon to the sound of a band and children waving Libyan flags and chanting "We want Gaddafi" in English, the Reuters news agency reports.
Col Gaddafi - who was last seen on state television meeting tribal leaders on 11 May - was not among the dignitaries who greeted him.
AnalysisOne of Col Gaddafi's advisers admits the South African president's visit may be their last chance of a diplomatic way out, but says there's no possibility of the man they call the Brother Leader stepping aside - as both Nato and the rebels are demanding.
There is support for the alliance, though, on the streets of Tripoli, even as life gets tougher. Petrol shortages are now so severe that people report queuing for up to five days.
No-one likes being bombed, said one resident and anti-Gaddafi campaigner, but we need Nato to get rid of him.
But his opponents are still too scared to protest openly in Tripoli.
It also rejected as "misleading" reports that their talks would focus on agreeing an exit strategy for Col Gaddafi.
South Africa voted for the UN Security Council resolution authorising the use of force to protect civilians in Libya despite the AU's concerns. Since then, Mr Zuma has joined other African leaders in accusing Nato of overstepping its mandate and calling for an end to the bombardment.
The BBC's Andrew North in Tripoli says some hope Mr Zuma's charm and personal relationship with Col Gaddafi will make a deal possible.
But the prospects for this peacemaking bid look just as thin as last time, our correspondent says.
An African Union "roadmap", which was drawn up in February and called for an immediate ceasefire, was swiftly rejected by both the TNC and Nato because it did not call on Col Gaddafi to step down.
His supporters insist there is no possibility of him either leaving office or Libya, and point out that even if he did he could then face being arrested and taken to the International Criminal Court at The Hague on war crimes charges, our correspondent adds.
Libya - Key diplomatic initiatives22 Feb - Arab League suspends Libya
26 Feb - UN Security Council resolution 1970 imposes sanctions on Col Gaddafi and his family, and refers crackdown to International Criminal Court
10 Mar - France recognises rebel Transitional National Council as sole representative of Libyans
17 Mar - UN Security Council resolution 1973 authorises no-fly zone over Libya and use of "all necessary measures" to protect civilians
29 Mar - Governments and organisations agree at meeting in London to set up Libya Contact Group to co-ordinate efforts in post-Gaddafi Libya
10 Apr - Col Gaddafi accepts African Union's "roadmap" for ending conflict after visit by South African President Jacob Zuma; rebels reject plan as it does not require Col Gaddafi to step down
5 May - Ministers from Contact Group agree in Rome to set up non-military fund to help rebels
16 May - ICC's prosecutor seeks arrest of Col Gaddafi, his son Saif al-Islam, and intelligence chief Abdullah al-Sanussi for crimes against humanity
27 May - G8 leaders call on Col Gaddafi to go
"It is with this in mind that we would like to reconfirm that the basis of any consideration for the resolution of the Libyan crisis is the removal of the main reason for this crisis, Col Gaddafi. As such, there is no room for negotiation until his departure and the departure of his regime."
On Monday, rebel spokesman Guma al-Gamati told the BBC World Service that he believed Mr Zuma's visit would make a difference as Col Gaddafi was far weaker and more isolated than he was last month.
"The people around him and the aides and people who are fighting for him are diminishing; some are deserting," he added.
Nato Secretary General Anders Fogh Rasmussen echoed the comments, telling a meeting of the alliance's parliamentary assembly: "Gaddafi's reign of terror is coming to an end. He is increasingly isolated at home and abroad. Even those closest to him are departing, defecting or deserting."
Mr Zuma's visit comes days after the UK and France announced they were sending attack helicopters to join the Nato effort, as the alliance attempts to break the deadlock which has left the rebels in control of eastern Libya and the government running most of the west.
The UK also said it could start using "bunker-busting" Paveway III bombs, capable of penetrating reinforced buildings.
Libyan state media said on Monday that Nato aircraft had killed 11 people at civilian and military sites in Zlitan, 50km (30 miles) west of Misrata.
Al-Arabiya meanwhile reported that a group of 120 Libyan army officers had defected and travelled to Rome. A rebel leader told the AFP news agency that eight senior officers, including four generals, were among them and would shortly address the media in Rome.