KABUL - Taliban fighters attacked part of a US military outpost in the eastern Afghan province of Nangarhar on Monday, officials said, less than a week after targeting a Polish base in a nearby province.Attacks on Afghan security forces and civilians have also mounted this week killing dozens and adding to fears the draw-down of foreign troops, most of whom are due to leave the country by the end of next year, is allowing insurgents to regain lost territory."In a parking lot used by Americans there was a huge blast and after that a massive attack... A number of our heroes are fighting," said Taliban spokesman Zabihullah Mujahid in an email.The attack prompted police to shut part of a highway between the provincial capital and Torkham district near the border with Pakistan, a security source said. The base itself had not been penetrated.No casualties had been reported, provincial spokesman Ahmad Zia Abdulzai said.