Secretary-General Ban Ki-moon says two peacekeepers serving in the joint United Nations-African Union force in Sudan's troubled Darfur region have been killed in an attack.

The U.N. chief gave no details about Thursday's attack but said it has been "a bloody October for U.N. peacekeeping" with 14 peacekeepers killed in Darfur, Mali and the Central African Republic. This amounts to "nearly one per day," he said.

"This highlights the exceptionally challenging environments in which today's peacekeepers operate," Ban told reporters.

Rebel groups have been fighting in the vast western region for more than a decade and the security situation has worsened this year.