A contentious weeklong referendum in Sudan was marred by violent clashes between Arab nomads at the North-South border on Sunday, according to leaders in the contested Abyei region. At least 23 people were killed in the fighting. The vote, in which the southern part of the country is expected to vote for independence, depriving the north of the country's oil reserves, only reached its second day before the violence broke out. President Obama has warned leaders from both sides not to employ "proxies" to sabotage the other side, and both sides have denied responsibility for the violence. The Abyei region is situated between the North and South and was promised its own referendum on which side to join. But leaders couldn't agree on how to hold the vote, so it never happened.