Senior US State Department official Jeffrey Feltman is in Tripoli for talks with Libya's new leaders.
Meanwhile, Saadi Gaddafi, one of Muammar Gaddadi's sons, has been placed under guard in Niger.
Feltman, the assistant secretary of state for Near Eastern affairs, is the highest ranking US official to visit the Libyan capital since its capture from Muammar Gaddafi's forces on August 23.
"Mr Feltman is here and is scheduled to meet (National Transitional Council) chairman Mustafa Abdel Jalil," NTC spokesman Mahmud Shammam said.
A US advance team headed by embassy number two Joan Polaschik arrived in Tripoli on Saturday to make preliminary contacts, and assess the material condition and security of the embassy, the State Department said.
The team reported that "the water situation, the electricity situation, essential services, do seem to be coming back to normal," spokeswoman Victoria Nuland said.
Reopening the US diplomatic mission with ambassador Gene Cretz at the helm should be a question of weeks not months, Nuland added.
The NTC chief arrived in Tripoli from his wartime base in Libya's second-largest city Benghazi on Saturday and was given a hero's welcome.
A crowd of hundreds mobbed Abdel Jalil at the airport and he had to be protected by a thick human chain.
The interim leader has said that he has only returned to the capital on a temporary basis and that he will only set up base permanently Tripoli once the liberation of Libya from pro-Gaddafi forces is complete.
SAADI GADDAFI 'UNDER GUARD'
The toppled despot's son Saadi was placed under guard in the Niger capital today after fleeing Libya over the weekend but NATO acknowledged it has no idea where his fugitive father is holed up.
The alliance stressed that the deposed leader is not a target in the daily bombing campaign it has kept up against his remaining forces, which still control a swathe of the Mediterranean coast around his hometown of Sirte as well as a string of Saharan oases.
Our understanding is, like the others, he's being detained in a state guest house- US State Dept
Saadi Gaddafi, 38, the third of Gaddafi's seven sons, is among 32 officials of the ousted regime, three of them top generals, who have fled through the desert to neighbouring Niger this month.
He was flown into the capital Niamey late yesterday after being put on an air force Hercules C-130 transport plane from the northwestern town of Agadez, Nigerien officials said.
Washington accepted Niamey's assurances that Saadi Gaddafi, who commanded an elite army unit after a brief career as a professional footballer in Italy, was in the custody of Nigerien security forces.
"Our understanding is, like the others, he's being detained in a state guest house," State Department spokeswoman Victoria Nuland said.
"It's essentially a house arrest in this government facility, is our understanding," she added.
NATO said it had no idea whether Gaddafi himself had also fled his country.
Colonel Roland Lavoie, spokesman for NATO's Libya mission, said the alliance had received, at "various points" in the conflict, intelligence confirming that Muammar Gaddafi was still in Libya, but that his whereabouts were now a mystery.
"To be frank we don't know if he has left the country," Lavoie told reporters on Tuesday.
Lavoie stressed that it was not NATO's mission to hunt down the fugitive former strongman.
"We are not in the business of targeting or chasing Gaddafi," he said.
But the Canadian colonel said Gaddafi's options were increasingly limited as forces loyal to the NTC close in on his remaining strongholds,
He said advances by rebel forces in the past two days had cut pro-Gaddafi forces in the strip of territory between his hometown of Sirte on the Mediterranean coast and the desert town of Bani Walid from those in the southern oases of Waddan and Sabha with their access to Libya's desert border.
NATO said today that its aircraft had hit nine targets around Sirte, seven around Waddan and one around Zillah, another oasis town to its east.
A trickle of civilians fleeing Sirte arrived in the NTC-held town of Sadada to its west on Wednesday, an AFP correspondent reported.
They know war is coming towards Sirte so they are looking for a safe place- NTC Commander
"They know war is coming towards Sirte so they are looking for a safe place," said NTC commander Omran Abdul Salam.
A top-level African Union team was meanwhile meeting in Pretoria to discuss ways to press the pan-African organisation's call for an "inclusive" government in Libya.
Gaddafi was a major player in the AU, bankrolling many of of its poorer members, and the organisation has so far refused to give Libya's seat to the new leadership in Tripoli.
South African Foreign Minister Maite Nkoana-Mashabane said the AU believed "the NTC is an opportunity to create an all-inclusive interim government to prepare for a democratic government through a democratic election, preceded by an interim constitution."