BENGAHZI, Libya, Aug 17 (Reuters) - Security forces in eastern Libya said on Thursday they had arrested the commander of an elite forces unit who is sought by the International Criminal Court (ICC) for allegedly executing dozens of prisoners.

The general command of the Libyan National Army (LNA), the force that controls most of eastern Libya, said the commander, Mahmoud al-Werfalli, was being investigated by a military prosecutor.

"We inform you that the defendant in your judicial case, Mahmoud al-Werfalli, is under investigation for the cases against him by the general military prosecutor and is now under arrest," the LNA said in a statement.

Werfalli is wanted for allegedly executing dozens of prisoners in the spring and summer of this year, toward the end of a three-year LNA campaign against Islamists and other opponents for control of the eastern city of Benghazi.

The Special Forces is a powerful elite unit nominally under LNA control that joined the Benghazi campaign in its early stages.

The LNA commended the ICC on its "efforts to achieve stability and social security."

"We announce our readiness to cooperate with you in informing you of the result and course of the judicial case," the LNA said.

However, the statement gave no indication that the LNA would be prepared to hand Werfalli over to the ICC, which issued an arrest warrant for him on Tuesday.

A special forces spokesman had earlier dismissed the warrant, saying the ICC should instead be seeking to arrest the LNA's opponents.

The LNA, led by Khalifa Haftar, is the dominant force in eastern Libya, but has opposed a United Nations-backed government in the capital, Tripoli. It declared victory in its Benghazi campaign in July.

(Reporting by Ayman al-Warfalli, writing by Aidan Lewis, editing by G Crosse) ((Aidan.Lewis@thomsonreuters.com; +216-2985-0352))