US President Barack Obama has defended the first military intervention of his presidency, insisting US involvement in Libya will be limited. He said US participation in the coalition had saved "countless lives", but that overthrowing Muammar Gaddafi by force would be a mistake. Delegates from dozens of countries are gathering in London for a conference on the future of Libya. The rebel advance there has been halted near Col Gaddafi's birthplace, Sirte. Anti-Gaddafi forces had made rapid progress westwards from their stronghold in Benghazi in recent days, greatly aided by international air strikes. But rebel fighters said pro-Gaddafi forces had used heavy weaponry to check their advance some 50km (30 miles) east of Sirte, says the BBC's Ben Brown in Ajdabiya. In eastern Libya, rebel radio has been urging more people in the west of the country to join the anti-Gaddafi uprising.