TUNIS (Reuters) - Tunisian President Beji Caid Essebsi has said U.S. surveillance drones are flying over the Tunisian-Libyan border to ward of attacks by Islamic State, after lawmakers called on his government to clarify U.S. reports on such a mission.

In Tunisia, any U.S. military presence on its soil is a sensitive topic. Opponents say it would be a breach of sovereignty. Last month U.S. government sources said U.S. surveillance drones had begun flying into Libya from a Tunisian air base.

But Essebsi, in a television interview broadcast late on Tuesday, said the measure was needed to avoid cross-border attacks by militants such as an Islamic State assault on the Tunisian town of Ben Guerdan in March.

The drones being used were unarmed, Essebsi said, and in a repeat of previous denials, added there was no U.S. base in Tunisia.

Asked if U.S. drones were flying missions at the border, Essebsi told local channel Elhiwar Ettounsi, "Yes, and it was at our request."

"Our agreement with the U.S. was to share intelligence information," he said. He did not say whether the drones were flying across Libyan territory.

Islamic State exploited the chaos of post-uprising Libya to establish a foothold there, using it as a base for attacks Tunisia and Egypt. The perpetrators of several militant attacks in Tunisia in the past two years were trained in Libya.

The United States is fighting the group's Libyan arm with air strikes launched from its ships in the Mediterranean and armed drones launched from Sicily.

Washington's use of armed drones against militants in countries such as Yemen and Pakistan has been criticised for its toll on civilians, and some opponents question its legality under international law.

Essebsi said there was a fear of attacks from abroad.

"We do not have clear information, and we should act instead of waiting until another Ben Guerdan," he said.

Dozens of Islamic State fighters stormed through the Tunisian town of Ben Guerdan near the Libyan border in March, attacking army and police posts.

Tunisian forces repelled the attack, but at least 53 people were killed in the raid, including civilians.

Essebsi said the surveillance drones would be given to Tunisia after training conducted by 70 U.S. soldiers. He did not say how many drones were being used or how long the training would last.

(Reporting By Tarek Amara; Editing by Aidan Lewis and Raissa Kasolowsky)