WASHINGTON - US military forces carried out an operation on Monday against al-Shabaab militants in Somalia, a US Department of Defense spokesman said."We are assessing the results of the operation and will provide additional information as and when appropriate," Pentagon spokesman Rear Admiral John Kirby said in a statement.No further details about the operation in the African country were immediately available.Al-Shabaab is an Islamist group affiliated with al-Qaida that wants to impose its own strict version of Islam in Somalia. It ruled most of the southern region of Somalia from 2006 until 2011, when African peacekeeping troops marched into the capital, Mogadishu.African and Somali forces have regained several towns this year, but rebels still hold other centers and tracts of countryside.On Saturday, the US also carried out air strikes against Islamic State fighters near the besieged Shi'ite town of Amerli in northern Iraq and airdropped humanitarian aid to civilians trapped there, the Pentagon said.President Barack Obama authorized the new military action, broadening US operations in Iraq amid an international outcry over the threat to Amerli's mostly ethnic Turkmen population.Aid was delivered by US aircraft as well as planes from Britain, France and Australia, signaling headway in Obama's efforts to draw allies into the fight against Islamic State.Iraqi army and Kurdish forces closed in on Islamic State fighters on Saturday in a push to break the Sunni militants' siege of Amerli, which has been surrounded by the militants for more than two months.Armed residents of Amerli have managed to fend off attacks by the Islamic State fighters, who regard its majority Shi'ite Turkmen population as apostates. More than 15,000 people remain trapped inside."At the request of the government of Iraq, the United States military today airdropped humanitarian aid to the town of Amerli, home to thousands of Shia Turkmen who have been cut off from receiving food, water, and medical supplies for two months by ISIL," Rear Admiral John Kirby, the Pentagon press secretary, said, using an alternative name for Islamic State."In conjunction with this airdrop, US aircraft conducted coordinated air strikes against nearby ISIL terrorists in order to support this humanitarian assistance operation," he said, adding that a key objective was to prevent a militant attack on civilians in the town.He said the operations would be "limited in their scope and duration" as required to protect Amerli's population.When Obama ordered the first air strikes and air drops in Iraq earlier this month, he justified the military operation in part to prevent a humanitarian catastrophe for thousands of ethnic Yazidis trapped by Islamic State militants on Sinjar mountain in northern Iraq.In mid-August, he declared that the militant siege there had been broken.