PARIS - French President Nicolas Sarkozy, flanked by British Prime Minister David Cameron, urged Syrian opponents of President Bashar Assad on Friday to unite so that the outside world could help them overthrow him.Sarkozy said the lack of unity among opposition groups was as much of a hurdle to resolving the crisis than opposition within the UN Security Council to taking action and warned that without a credible alternative the uprising would fail.Cameron also said the lack of a unified Syrian opposition was limiting what the outside world could do to help, but said France and Britain were working together to see how they could better help opponent groups.On Thursday, 137 states voted in favor, 12 voted against and 17 abstained at the UN assembly on a resolution endorsing an Arab League plan telling Assad to step down. Russia and China voted against, after vetoing a similar Security Council text on Feb. 4."The principle obstacle ... is not just the blockage at the UN In Libya we couldn't have had the revolution without the Libyans and we won't be able to have a Syrian revolution without the Syrian opposition making enough effort to unite that we can support them more," Sarkozy said."We will not accept that a dictator massacres his people, but the revolution will not be led from outside. Like in Tunisia, Egypt and Libya, it must be led from the inside," he told a news conference following a Franco-British summit.Assad is showing no sign of heeding calls to halt the repression of the 11-month uprising against his rule. His forces on Friday resumed pummeling opposition strongholds in the city of Homs, which has been under fire for two weeks.