South Sudan has been fighting over disputed territory with Sudan ever since the two split apart last July.
The fighting has intensified in recent weeks.
Now, southern military forces are occupying an oil-rich town that was under northern control.
And South Sudan's president says he won't withdraw them.
The south says it's responding to previous attacks by the north.
Tensions are rising sharply and there's concern that an all-out war between South Sudan and Sudan could break out.
BBC South Sudan correspondent Nyambura Wambugu is in Juba, the capital of South Sudan.