PRETORIA, South Africa (AP) - President Bush pledged Wednesday that he will not overextend U.S. armed forces even if he sends troops to join a peacekeeping force in Liberia.

Bush renewed his pledge to "be involved" in Liberia, where civil war has dragged on for years and conditions in Monrovia have become desperate amid a political stalemate involving President Charles Taylor.

South African President Thabo Mbeki pressed Bush on whether the United States planned to play a role in the crisis. "I said, 'Yes, we'll be involved, and we're now discussing the extent of our involvement," Bush told reporters during a joint news conference with Mbeki.

The United States already has tens of thousands of soldiers in Afghanistan, Iraq and Kosovo. Bush said that whatever he decides to do about Liberia, "we won't overextend our troops."

Mbeki said that the military burden in Liberia peacekeeping "really ought to principally fall on us as Africans."

The United States has trained battalions of African troops, Bush said, and "helping people help themselves" was one method of ensuring the U.S. military would not become stretched too thin there.

"It's in our interest that we continue that strategy so that we don't get overextended," Bush said.

Bush did not say whether he will deploy troops to Liberia. He promised that "we will work closely with the United Nations and the Economic Community of West African States to enforce the cease-fire, to see to it that Mr. Taylor leaves office so there can be a peaceful transition in Liberia."

Bush wants African leaders to put more pressure on President Robert Mugabe of Zimbabwe to step down and hold new elections. Mbeki has insisted that he will not pressure Mugabe.

About half of Zimbabwe's population faces starvation. Dozens have been killed in state-orchestrated political violence. Thousands more have been beaten, jailed, raped or tortured for their views.

Wednesday, Bush backed Mbeki's efforts in Zimbabwe.

"The president is the one who is most involved" in mediating troubles in Zimbabwe," Bush said. He believes he's making good progress. ... I don't have any intention of second-guessing his tactics."

Bush's visit to Africa began Tuesday in the Atlantic port city of Dakar, the capital of Senegal. He met privately with Senegalese President Abdoulaye Wade, then with a larger group that included Wade and the heads of seven other West African democracies.

Bush and his party then rode in Wade's presidential yacht to Goree Island for a tour of a centuries-old slave house where hundreds of thousands of Africans were bought and sold like cargo.

In a speech on the island, Bush stopped short of issuing the blanket apology for slavery that some civil rights advocates had sought. But he acknowledged that the scars of slavery still sting American society.

"But however long the journey, our destination is set: liberty and justice for all," the president said.