US President Barack Obama has hailed the start of a landmark referendum on independence in Southern Sudan. He said the week-long vote - which is expected to result in Africa's largest country being split in two - represented a "new chapter in history". The poll was agreed as part of the 2005 deal that ended a two-decade civil war. The mainly Muslim north has promised to allow the potential new country, where most people are Christian or animists, to secede peacefully. Mr Obama said in a statement: "After 50 years of civil wars that have killed two million people and turned millions more into refugees this is the opportunity before the people of Southern Sudan."He added that the action of Sudanese leaders would help determine whether Sudanese people move "toward peace and prosperity, or slide backward into bloodshed". Sudanese President Omar al-Bashir has promised to respect the outcome, but warned an independent south would face instability. Southern Sudanese voters are faced with two symbols on the ballot paper - a single hand for independence or two clasped hands to remain one country.South Sudanese leader Salva Kiir, casting his ballot on Sunday, urged people to "be patient", in case they were not able to vote on the first day of polling.Turnout in the referendum will be important, as the 2005 peace agreement stipulates that for the vote to be valid, 60% of the 3.8 million registered voters must take part.