Shokri Ghanem, who oversaw Libya's oil and gas sector, is the second most senior official to quit and rebels said the defection showed that the end is nearing for Qaddafi almost four months into a rebellion against him.
But a government spokesman in Tripoli played down the significance of Ghanem's departure. "This is a country, a state, a government, not just one person," Mussa Ibrahim told Reuters.
He said the government would be represented at the meeting of the Organization of the Petroleum Exporting Countries in Vienna on June 8. "I don't have a name yet but we'll have somebody," he said.
Ghanem appeared on Wednesday at a news conference in Rome after leaving Libya over a week ago.
"I have been working in Libya for so many years believing that we can make a lot of reform from within. Unfortunately this became not possible, especially now, when we see the spilling of blood every day in Libya," Ghanem said.
An executive with Libya's state-owned National Oil Corporation, Mosbah Ali Matoug, took Ghanem's place on Thursday at a meeting of the Gas Exporting Countries Forum in Cairo.
Meanwhile, Libyan rebel officials are in contact with top oil companies that operate in the North African country but no new contracts are being drawn up, a source in the rebel leadership said.
Information and pricing agency Platts reported this week that the rebel National Transitional Council (NTC) was in preliminary talks with Italy over a possible renegotiation of a production-sharing deal for Libya's Bouri offshore field.
It said the talks might result in the rebels taking charge of the stake in the field owned by Libya's state National Oil Corporation, which operates Bouri in association with Italy's Eni.
"We are having talks with top companies that operate in Libya, but we are not making new contracts," the source in the Benghazi-based national council told Reuters.
Officials in the rebel leadership have played down the likelihood that they will tear up Libya's revenue sharing agreements with foreign oil majors, saying they will respect contracts signed by NOC.
Libya was producing around 1.6 million barrels per day of oil and exporting some 1.3 million bpd before the unrest.
Eni, the biggest foreign oil company in Libya, is trying to find a way to make exporting oil from Libya work without running afoul of sanctions, a source familiar with the matter told Reuters in Italy.
