They told a news briefing that 86 of the 93 people reported to have had the deadly and incurable disease had died. No cases had been reported in Kinshasa.
"We have just found where she [the nurse] is," said Bompenda Bonkumo, head of a national committee tracking the progress of the disease.
Officials had the necessary equipment to find out whether she had the disease, he said. "We will be able to advise on her state of health tomorrow."
A riverboat captain, also placed in isolation in Kinshasa after arriving from Kikwit, where the outbreak started, had been given a clean bill of health, officials said.
A statement from the committee said that 93 cases had been registered by Tuesday, of whom 72 had died in hospital and 14 outside hospital.
Most of the cases were in Kikwit, 500km (310 miles) from Kinshasa. Diplomats said the ambassadors of Belgium and the United States were due to visit Kikwit today with the Zairean Health Minister.
The Kinshasa Governor, Bernadin Mungul Diaka, said earlier that the nurse had absconded from the university health centre.
The virus, for which there is no vaccine nor cure, spreads through contact with blood or body fluids and kills by causing uncontrollable bleeding. Ebola is one of the most lethal diseases known.
Authorities in Kinshasa have tightened roadblocks on highways from the outbreak zone and issued leaflets and graphic posters telling members of the public not to touch corpses or open wounds.
More governments started screening or monitoring passengers arriving from Zaire or elsewhere in Africa.
The committee tracking the epidemic - which broke out in Kikwit last month and has spread to at least four neighbouring towns - had asked for quarantine for 25 foreign journalists who visited Kikwit on Sunday but no action has so far been taken.