NEW YORK – The United States and the U.N.'s new Syria mediator grappled for a new strategy Tuesday toward stopping 18 months of brutal government crackdowns and civil war in the Arab country as President Barack Obama again called for the end of Syrian President Bashar Assad's regime.

Secretary of State Hillary Rodham Clinton and Lakhdar Brahimi spoke for an hour on the sidelines of the U.N. General Assembly, according to a senior U.S. official, charting paths that might help unite Syria's opposition and engineer a peaceful transition away from the four-decade Assad regime. Their talks came just hours after Obama told world leaders that Assad must leave "so that the suffering of the Syrian people can stop and a new dawn can begin."

"In Syria, the future must not belong to a dictator who massacres his people," Obama said. "If there is a cause that cries out for protest in the world today, it is a regime that tortures children and shoots rockets at apartment buildings."