U.S. helicopters flew into Pakistan's militant-infested border region, but returned to Afghanistan after troops and tribesmen opened fire, intelligence officials said Monday. Washington denied the account. The alleged incident late Sunday threatened new rifts between the United States and its key ally against terrorism days after a truck bomb struck a luxury hotel in Islamabad, killing 53 people. Late Monday, Dubai-based TV channel Al-Arabiya said it had received a tape from a shadowy group calling itself "Fedayeen Al-Islam" - Arabic for "Islam commandos" - claiming responsibility for the bombing and urging Pakistan to end cooperation with Washington. In the northwestern city of Peshawar, gunmen kidnapped Afghanistan's ambassador-designate Monday and killed his driver. Pakistan is under growing U.S. pressure to act against al-Qaida and Taliban insurgents sheltering in its border region and blamed for rising attacks on coalition troops in Afghanistan and suicide bombings in Pakistan. U.S. officials believe that al-Qaida's leaders, including Osama bin Laden, are hiding somewhere along the border. A series of suspected U.S. missile strikes into the lawless region and a Sept. 3 raid by U.S. commandos said to have killed 15 people have highlighted U.S. impatience and angered many Pakistanis. Two intelligence officials said on condition of anonymity because they were not authorized to speak to the media said two U.S. helicopters crosse ...