South Sudan's military is accusing the government of Sudan of aiding a new rebel offensive against South Sudanese government forces in the state of Upper Nile, where oil fields are now threatened by fresh fighting.

South Sudanese military spokesman Col. Philip Aguer told The Associated Press on Friday that the recent defection in Upper Nile state of a key general to the rebel side was likely tied to incentives from Sudan's government.

It's not the first time South Sudan is accusing its neighbor of supporting rebels, charges routinely denied by Khartoum.

Fighting has recently escalated in the states of Unity and Upper Nile, where the last remaining functional fields of Paloch are threatened.

Doctors Without Borders described an "alarming humanitarian situation" in the country as aid workers leave battle zones.