LUBUMBASHI, Zaire: Now that Zaire's summit-on-the-sea has ended in an apparent stalemate, Laurent Kabila is back to his preferred form of diplomacy: the ultimatum. From his headquarters in Lubumbashi, Kabila has given Mobutu Sese Seko eight days to yield to the rebel alliance or "be chased from the power." Fiercely denying a U.N. envoy's statement that he had agreed to a cease-fire with the ailing Mobutu, the rebel leader is giving his troops quick marching orders toward the capital. As he attempts to get Mobutu to resign before the soldiers arrive, Bill Richardson began casting about for ways to ensure Kabila's cooperation. The U.N. ambassador headed to Botswana to meet with Ugandan President and Kabila mentor Yoweri Museveni, who may have the leverage over Kabila that Richardson lacks. Though their meeting ended without a handshake, Kabila and Mobutu agreed to meet again in ten days. It is unclear how that date would fit with Kabila's ultimatum--or the rapid advance of his troops. Senior State Department officials say that parts of Kabila's army could be in Kinshasa by the end of this week.