The tripartite disarmament effort, which had been threatened by a rift between the IAEA and Washington over who would oversee the work, was on track one month after Libyan leader Moamer Kadhafi pledged to give up his nuclear, chemical and biological weapons capabilities.
Britain and the United States had agreed with the watchdog, the International Atomic Energy Agency (IAEA), on Monday in Vienna to let the UN agency oversee Libya's atomic disarmament, but for British and US experts to carry out the removal and destruction of equipment.
The IAEA team, the first UN inspectors in the country since IAEA director Mohamed ElBaradei visited Libya in December, arrived around 2:00 am Tuesday (0000 GMT), the diplomats said.
In Washington, US Secretary of State Colin Powell confirmed that a team of US experts was in Libya.
He said the team was headed by Donald Mahley, the State Department's special negotiator for chemical and biological arms control issues, and that he expected to see quick results on verification.
But he said he could not provide a timeline on when the process might be complete "until the work is done on the ground."
Tripoli had announced on December 19 that it was abandoning attempts to develop biological, chemical and nuclear weapons, a surprise that came after months of secret negotiations between Tripoli, London and Washington.
"By the end of the week, the IAEA should have about eight inspectors in Libya," a Western diplomat close to the IAEA told AFP.
He said the IAEA wanted to "go to the roots" of the Libyan program and to verify documents already supplied by the Libyan government and provide an "inventory" of the Libyan nuclear program.
He said there was "no drama" for the IAEA in cooperating with the Americans on the ground.
The IAEA and the Americans "have good channels of communication. Everyone knows his role," the diplomat said.
The Americans and British are also working with the Libyans on disarming chemical and biological weapons capacities. The IAEA only deals with nuclear issues.
The Western diplomat said the IAEA "wants to establish how many centrifuges are there, where they came from," referring to equipment crucial in making the highly enriched uranium that is the basic ingredient for an atomic bomb.
The diplomat said that once the IAEA had completed this verification work "the Americans and British will be free to take the equipment out of the country."
"It's their job to take it out," he added.
He said the IAEA also wanted to know where the Libyan equipment came from and if this could help the agency decide how other countries, like Iran, had clandestinely acquired nuclear technology.
Britain and the United States are to provide logistical support to the inspection missions carried out by the IAEA, ElBaradei had said Monday, in explaining an agreement that ended a turf battle over who should take the leading role in verifying that Libya is making good on its promise to give up weapons of mass destruction.
The administration of US President George W. Bush had accused the IAEA, which is mandated to monitor adherence to the nuclear Non-Proliferation Treaty (NPT), of rushing into Libya.
The British-US team of about a dozen experts is in Tripoli planning how to destroy tonnes of mustard gas and how to evacuate any highly enriched uranium from Libya, a senior US official told the New York Times.
The illicit materials, the senior official said, would likely be shipped to a secure facility in Britain or the United States.
A Western official told The New York Times that US officials were considering opening an office in Tripoli for direct diplomatic contact between Libyan and US officials.
The State Department has said that US sanctions on Libya would not be lifted until Tripoli met the disarmament requirements for their removal and not before.