LIBREVILLE, Gabon (AP) _ Zairian President Mobutu Sese Seko agreed today to discuss a transition of power with the leader of a rebel army closing in on Zaire's capital, South Africa's deputy president said.

After Thabo Mbeki's announcement, Mobutu's plane left Gabon for Kinshasa. His return today to the Zairian capital _ delayed for several days _ dispelled rumors that he would go into exile rather than come back to face the rebels.

Mbeki said the meeting between Mobutu and rebel leader Laurent Kabila will take place Wednesday aboard a South African ship in Pointe Noir, Congo, where the two met a week ago.

``The meeting should conclude these negotiations. The substantial questions, the principal questions, must be resolved on Wednesday,'' Mbeki said after meeting with Mobutu and the presidential palace in Libreville.

``Those questions involve the transition; President Mobutu's place,'' said Mbeki. He said Kabila agreed to the meeting on Friday.

Since September, Kabila's rebel forces have seized three-fourths of Zaire, and he told reporters today at his headquarters in Lubumbashi that his troops were closing in on the capital. He acknowledged international pressure for him to halt the advance, but he gave mixed signs about whether he would comply.

``We are waiting. We want to solve this through peaceful dialogue,'' he said. But he also said that ``nobody has officially asked me to halt the offensive. We are continuing.''

On Friday, rebel foreign minister Bizima Karaha said Kabila was only meeting with Mobutu on Wednesday to secure the president's resignation. He said a proposal for Mobutu to turn power over to a transitional figure was unacceptable. The rebels demand that Kabila be allowed to assume power.

The 66-year-old dictator, dying of prostate cancer and losing the civil war, has agreed to long-delayed elections and acknowledged he is too ill to be a candidate. He ordered his army earlier in the week to prepare for elections.

In eastern and central Zaire, where the war has displaced Rwandan Hutu refugees who had been living there since 1994, the United Nations stepped up efforts to take them back to Rwanda.

U.N. officials said they were doubling the number of refugees to 4,000 that they move each day from camps in the Zaire's central jungle.

The world body announced its decision Friday in Geneva amid pressure from Zairian rebels and the Rwandan government. Rebels have told the United Nations it wants all 80,000 refugees in the region out of the country by the end of June. So far about 15,000 have been sent home.

The Rwandan Hutus fled their country fearing retaliation for a Hutu-led massacre of half a million people, mostly ethnic Tutsis. Most of the refugees have returned to Rwanda, where Tutsis now are in power. Remaining refugees have reported being attacked by the rebels, many of them also ethnic Tutsis.

In Kinshasa, Zaire's capital, pressure was increasing on Mobutu to cede power. For the first time, political parties within the parliament announced their support for Kabila.

But Mobuto also had some support as several hundred demonstrators gathered in front of the U.S. Embassy to call for his return.

In a sign of the rebels' increasing hold on the country, investors have flocked to their headquarters to discuss mining plans for the mineral-rich country.

A Canadian mining company said Friday it will make a $50 million downpayment to Zairian rebels to develop what may be the world's largest copper and cobalt deposits near Lubumbashi.

The Vancouver-based Tenke Mining Corp. contract, with a total value of $250 million, would be the largest single infusion of cash for the rebels since they began their push to oust Mobutu.

While Mobutu has insisted he will return to Zaire, Western diplomats and at least one Mobutu aide have said privately they believe the president might use his trip to Gabon as a stepping stone to exile in a third country.