Libyan rebels clashed Wednesday with Sudanese mercenaries fighting for Muammar al-Qaddafi near the border with Sudan, as President Barack Obama predicted the Libyan leader would be forced to step down if NATO keeps up its military campaign with the U.S. playing a key role.

Speaking at a news conference in London, Obama said the U.S.-led NATO coalition was engaged in "a slow, steady process in which we're able to wear down the regime forces."

"There will not be a let up in the pressure we are applying" on Qaddafi, Obama said. "I believe that we have built enough momentum that as long as we sustain the course we're on, he will step down."

Government spokesman Moussa Ibrahim reacted angrily to Obama's assertion, saying "Qaddafi's destiny, Qaddafi's future, is for the Libyan nation to decide."

"It would be a much more productive statement to say that the Libyan people need to engage in an inclusive peaceful democratic transparent political process in which they can chose the shape of their political system and the leaders of their system," he said.

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A rebel commander in southeast Libya, Ahmed Alzway, said rebel fighters fought off a Sudanese mercenary force 18 miles  west of the southeast oasis of Kufra. The rebel force pursued, dislodging the Sudanese fighters from a fortified position further out in the desert, Alzway said.

He said they also found documents indicating the fighters from were the Sudanese Justice and Equality Movement.

In previous clashes at the southern border, captured Sudanese mercenaries have said they belonged to JEM, a Darfur-based rebel group. It could not immediately be determined whether the Sudanese fighters belonged to JEM.

Qaddafi has long provided arms, training and vehicles to various rebel groups in Sudan.

Witnesses in Libya have reported African mercenary fighters shooting at protesters or being captured by anti-Qaddafi forces. Some were flown in to put down the rebellion, but most fighters were already in the war-torn country.

Qaddafi has used Libya's oil wealth to aid neighboring African nations, including Sudan, and to fund the transformation of the old Organization of African Unity into the African Union, which has helped resolve conflicts on the continent. In February, the AU condemned attacks on civilian protesters in Libya.

Meanwhile, in NATO airstrikes overnight, British aircraft hit four of Qaddafi's armored vehicles near the Libyan city of Zlitan, British military spokesman Maj. Gen. John Lorimer said in a statement. Tornado and Typhoon jets also destroyed a radar station in the coastal city of Brega during the Tuesday night raid.

Libya's deputy foreign minister, Khaled Kaim, called on the South African leader Jacob Zuma to push forward negotiations to end the three-month conflict when he arrives next week in the capital Tripoli.

Zuma is the highest-ranking politician to visit Qaddafi since the fighting began in Libya.

Kaim told The Associated Press the Qaddafi government is hoping Zuma will help arrange a cease-fire between Libyan government forces, NATO and the rebels, and oversee a transitional period.

"The idea is how to find a mechanism to implement the road map -- a halt to fire, reconciliation, national dialogue, and then we'll have a transitional period maybe for a year or two," Kaim said, referring to an African Union initiative the Libyan government has embraced, but which rebels have rejected.

Citing a deep mistrust of Qaddafi's regime and emboldened by NATO strikes, the rebels have insisted Qaddafi must leave power before any negotiations can take place.

Throwing into doubt the effectiveness of Zuma's visit, the South African leader will not be meeting representatives of the interim government based in the de-facto rebel capital of Benghazi.

Kaim said there was no need for Zuma to consult the interim government, saying they did not represent most of the rebel insurgents currently battling Qaddafi's forces. "They are nine people. They don't represent what is going on in Benghazi and other cities in the east," he said.