U.S. airstrikes have temporarily slowed the Islamist fighters' momentum, but are unlikely to stop its overall advances, a U.S. Defense Department official said on Monday, adding that the United States had no plans to expand the current air campaign.

U.S. Lieutenant General William Mayville speaks during a briefing at the Pentagon, Washington D.C., capital of the United States, Aug. 11, 2014. Mayville told reporters that the U.S. had conducted 15 targeted air strikes in Iraq that have slowed the Islamic State of Syria and the Levant's operation, but had no plans to expand the current air campaign. [Xinhua]

U.S. Air Force and Navy aircraft have carried out 15 airstrikes since President Barack Obama Thursday authorized the military to provide humanitarian assistance to refugees on Sinjar Mountain in northern Iraq and to conduct airstrikes to protect U.S. citizens and forces in and out Erbil, said William Mayville, director of operations for the Joint Staff, during a Pentagon briefing.

"These airstrikes, carried out by F-15E Strike Eagles, F/A-18 Super Hornets and MQ-1 Predators, have helped check the advance of ISIS forces around Sinjar and in the area west of Erbil," he said.

While U.S. airstrikes in northern Iraq have slowed ISIL's operational tempo and temporarily disrupted their advance toward Erbil, Mayville said the "strikes are unlikely to affect ISIS' overall capabilities or its operations in other areas of Iraq and Syria."

"I think, in the immediate areas where we have focused our strikes, we've had a very temporary effect," the lieutenant general said, adding that ISIS is still intent on securing and gaining territory throughout Iraq and it will continue to attack Iraqi and Kurdish security forces, Yazidis, Christians and other minorities.

However, he noted that currently there were no plans to expand the current air campaign beyond the current self-defense activities.

"Our current operations in Iraq are limited in scope to protect U.S. citizens and facilities, to protect U.S. aircraft supporting humanitarian assistance, and to assist in the breakup of?ISIS forces that have laid siege to the Sinjar Mountain," the Mayville said.

The United States and Britain have flown 14 humanitarian airdrops since Aug. 7 to Yazidi refugees in the Sinjar Mountain in Iraq, he said.

More than 310 bundles of food, water and medical supplies have been delivered to the refugees, providing about 16,000 gallons of water and 75,000 meals, he added.