WASHINGTON, Oct 26 (Reuters) - The U.S. intelligence community has assessed that Islamic State in Afghanistan could have the capability to attack the United States in as little as six months, and has the intention to do so, a senior Pentagon official told Congress on Tuesday. The remarks by Colin Kahl, under secretary of defense for policy, are the latest reminder that Afghanistan could still pose serious national security concerns for the United States even after it ended its two-decade-old war in defeat in August. The Taliban, which won the war, are enemies of Islamic State and have seen its attempts to impose law and order after the U.S. pullout thwarted by suicide bombings and other attacks claimed by Islamic State. They include bombings targeting the minority Shi"ite sect and even an Islamic State beheading of a member of a Taliban militia force in the eastern city of Jalalabad. In testimony before the Senate Armed Services Committee, Kahl said it was still unclear whether the Taliban has the ability to fight Islamic State effectively following the U.S. withdrawal in August. The United States fought the Taliban as well as striking groups like Islamic State and al Qaeda. "It is our assessment that the Taliban and ISIS-K are mortal enemies. So the Taliban is highly motivated to go after ISIS-K. Their ability to do so, I think, is to be determined," Kahl said, using an acronym for Islamic State in Afghanistan. Kahl estimated Islamic State had a "cadre of a few thousand" fighters. Acting Foreign Minister Amir Khan Muttaqi of the new Taliban government has said the threat from Islamic State militants will be addressed. He also said Afghanistan would not become a base for attacks on other countries. Kahl suggested al Qaeda in Afghanistan posed a more complex problem, given its ties to the Taliban. It was those ties that triggered the U.S. military intervention in Afghanistan in 2001 following al Qaeda"s Sept. 11 attacks on New York and Washington. The Taliban had harbored al Qaeda leaders. Kahl said it could take al Qaeda "a year or two" to regenerate the capability to carry out attacks outside of Afghanistan against the United States. Democratic President Joe Biden, whose supervision of the chaotic end to the war last summer has damaged his approval ratings, has said the United States will continue to be vigilant against threats emanating from Afghanistan by carrying out intelligence-gathering operations in the country that would identify threats from groups like al Qaeda and Islamic State. Kahl said the goal was to disrupt those groups so that Islamic State and al Qaeda don"t become capable of striking the United States. "We need to be vigilant in disrupting that," he said. Still, U.S. officials privately warn that identifying and disrupting groups like al Qaeda and Islamic State is extremely difficult without any troops in the country. Drones capable of striking Islamic State and al Qaeda targets are being flown in from the Gulf. Kahl said the United States did not yet have any agreement with countries neighboring Afghanistan to host troops for counterterrorism efforts. Reporting by Phil Stewart and Idrees Ali; editing by Jonathan Oatis Our Standards: The Thomson Reuters Trust Principles.