WASHINGTON/BERLIN (Reuters) - The United States has begun using a Tunisian air base to conduct surveillance drone operations inside Libya, the latest expansion of its campaign against Islamic State militants in North Africa, U.S. government sources said on Wednesday.

The unarmed drones have been flying out of Tunisia since late June and are now part of a U.S. air defense in support of Libyan pro-government forces fighting to push Islamic State fighters out of their stronghold in the Libyan city of Sirte, the officials said, speaking on condition of anonymity.

The use of the Tunisian base, which was first reported by The Washington Post, extends the U.S. military’s ability to gather intelligence on Islamic State in Libya, the U.S. sources said. Other locations in Africa where U.S. drones are launched, including Niger and Djibouti, are farther away.

In Tunis, a Tunisian defense official, speaking on condition of anonymity, denied that the drones were flying into neighboring Libya and said they were instead used for training Tunisian forces and protecting the country’s borders.

U.S. officials said the Air Force Reaper drones were being operated by U.S. personnel with Tunisian government approval from an existing base operated by Tunisia’s military and were currently used only for surveillance. But such unmanned aircraft are also capable of being armed.

“There are U.S. service members working with the Tunisian security forces for counter terrorism and they are sharing intelligence from various sources, to include aerial platforms,” said U.S. Army Colonel Mark Cheadle, spokesman for U.S. Africa Command.

Tunisia, a close U.S. counterterrorism partner, requested additional military equipment and training from Washington after deadly militant attacks last year and has been given more than $250 million in security assistance, Cheadle said.

Tunisian Defense Minister Farhat Horchani was cited by state news agency TAP as saying that Tunis has recently received reconnaissance planes and unmanned drone systems” for training and to “monitor the southern borders and detect any suspicious movements.”

He said the presence of foreign troops in Tunis was in the context of “international military cooperation” on mutual interests.

Forces aligned with Libya’s U.N.-backed Government of National Accord have been fighting for months to liberate Sirte from Islamic State, which took over the city amid factional infighting that emerged after the fall of Libyan strongman Muammar Gaddafi in 2011.

The pro-government forces have been bolstered by U.S. air strikes since August and coordination with small teams of Western special forces on the ground.