JUBA, Sudan  As voters continued flooding the polls on Monday for a landmark referendum on southern Sudan’s independence, officials said more than 40 people had been killed over the weekend in intense skirmishes in a contested area along Sudan’s north-south border.

The voting, which began Sunday, is proceeding jubilantly and remarkably smoothly, with high expectations and few serious complaints anywhere across southern Sudan. But if the referendum passes and the south breaks off from the north, the disputed border will become the next issue to resolve, and some fear that the specter of an all-out border war is rising.

Abyei, where skirmishes broke out Friday and Saturday, is considered the most combustible and intractable of all the disputed areas. Both the north and the south claim historical ties to it and are refusing to budge. Some Western analysts have called Abyei “Sudan’s Jerusalem.”