The highest-ranking official yet to break ranks with Libyan leader Moammar Gadhafi, Kusa arrived in London Wednesday via Tunisia, saying he was resigning his post as Libyan foreign minister.
British Foreign Secretary William Hague said Kusa had not fled to London on any deal.
"Musa Kusa is not being offered any immunity from British or international justice," Hague said. "He is voluntarily talking to British officials, including members of the British Embassy in Tripoli now based in London, and our ambassador, Richard Northern."
The New York Times reports that a second top official, Ali Abdussalam Treki, a former foreign minister and U.N. ambassador, fled Thursday to Egypt.
Especially Kusa's defection could help Western forces pinpoint strategic targets within Libya, observers say. He's being questioned by British authorities.
Despite the valuable information Kusa may provide, critics of the Libyan regime expect him to stand trial.
During the 1980s, he ran the Libyan foreign spy services and was involved in killing Libyans who had defected abroad. He's also believed to have had a hand in the 1988 bombing of an American passenger plane over Lockerbie, Scotland, an attack that killed 270 people, most of them Americans.
A prosecutor with the International Criminal Court in The Hague earlier this month placed Kusa on a list of Libyan government officials suspected of ordering crimes against humanity in connection with the government-ordered attacks on demonstrators.
While Hague called Kusa's departure a sign that the Gadhafi government was "fragmented, under pressure and already crumbling from within," analysts are more careful. They don't expect Gadhafi to be discouraged by the recent defections as his very loyal inner circle consists of mostly family members.
Meanwhile in Libya, the rebels are trying to regroup after a failed advance on the oil port of Brega.
Western observers say the rebels lack a clear structure, equipment and experience to take over the country.
A top rebel military commander, Maj. Gen. Suleiman Mahmoud, admitted to the BBC that the rebels needed outside assistance to win the fight against Gadhafi's forces.
"Our problem (is) we need help -- communication, radios, we need weapons," he said.