* Army clashes with rebels in Jonglei, Unity states
* Civilians say fear more ethnic-based killings
* 60,000 people seeking refuge in U.N. bases (Adds comments from U.N., reports of fighting in Unity and Jonglei states)
By Carl Odera and Aaron Maasho
JUBA, Dec 29 (Reuters) - South Sudan accused rebels on Sunday of using a feared ethnic militia in renewed attacks despite a government ...