Mr Kabbah also told a news conference at Freetown's Lungi airport town, where he is sheltering from the fighting, that he was prepared to free Mr Sankoh, who is under a death sentence for treason, if he agreed to certain conditions. One of the main rebel demands has been for the freeing of their leader. A rebel commander, Sam Bockarie, said on Wednesday the offensive would continue until Foday Sankoh was freed.
Amid confusion about the success of the Nigerian-led Ecomog forces' drive against the rebellion, Robin Cook, the Foreign Secretary, accused neighbouring Liberia of backing efforts to topple Mr Kabbah. "We believe there is credible evidence that they are supporting the rebels. If they are giving any support that must stop immediately," Mr Cook told BBC Radio 4 that 50 Britons remained in Sierra Leone.
In his radio interview, Mr Cook said Britain had invested pounds 30m in Sierra Leone since President Kabbah was restored to power.
People in Freetown said by telephone that Ecomog troops appeared to have retaken State House and the main prison, which was captured by rebels on Wednesday. Ecomog troops, backed by jets, were also reported to be advancing on the rebel-held Eastend district of Freetown.
Earlier this week Britain committed pounds 1m of financial and logistical support to Ecomog, the West African peace and intervention force, for its work in Sierra Leone. Last year the Sandline British mercenary company played a role in reinstating President Kabbah, who was ousted by rebels under Major Johnny Paul Koroma in May 1997. Sandline's shipments of arms and personnel were found to be in breach of international sanctions.
The Sierra Leone Information Minister, Julius Spencer, talking on the pro-government Radio Democracy network, urged civilians to stay indoors to avoid being shot by Ecomog.
In Britain, a Foreign Office spokesman said Mr Cook had written to the Liberian president, Charles Taylor, a week ago about his purported support for the rebels, warning that Liberia would "damage its standing with international financial institutions" unless it ended its backing.
On 29 December Liberia denied accusations by the United States and six West African nations that it was aiding the rebels but confirmed it knew of mercenaries helping the rebels.
Mr Taylor has accused President Kabbah's government of hiding rebels trying to oust him. But in the past year, there have been extensive reports of torture and other human rights abuses and atrocities by the Sierra Leonean rebels camped in the north of the country.
Meanwhile, Kayode Fayemi, director of the Centre for Democracy and Development think-tank, which is based in London, said yesterday the Nigerian army did not have the option of cutting and running after sending nearly a fifth of its ground troops to defend the elected President Kabbah.
"Ecomog cannot let this lie low for its own integrity, Nigeria cannot let it," he said. "I think the response on the part of the Nigerians for now would be to send in more battalions - but this is a no-win situation for them. Even if they do they would have to back this up with a political agenda."
Military analysts blame the Sierra Leone debacle on inadequate motivation, poor intelligence and an even poorer response to intelligence because of the belief that a rag-tag bunch of bush fighters could not outgun a force of at least 10,000 backed by air power.
"Officers were more concerned about personal perks than they were about the rebel threat. The men were complaining they were not being paid and had no idea what they were supposed to die for," one analyst said.