South Sudan's president accused Sudan of declaring war on his nation as tensions between the two neighbors intensified, sparking fears of a return to conflict over unresolved oil and border issues.

President Salva Kiir made the comments during a visit to Beijing, marking the latest in fiery war rhetoric that has drawn alarm and condemnation from the international community.

This trip comes  at a critical time "because our neighbor Khartoum has declared war" on South Sudan, he said Tuesday.

China - a crucial trading ally to both nations - urged restraint and negotiations on outstanding issues between the two rivals.