Gunmen on Wednesday killed six people, including the head of a Japanese aid agency, in an attack on their vehicle in Jalalabad, eastern Afghanistan, officials said. The ambush came a week after a grenade attack on a UN vehicle in Kabul heightened fears for those performing humanitarian work amid one of the world's longest-running conflicts. Tetsu Nakamura, head of Peace Japan Medical Services, was involved in rebuilding Afghan irrigation and agriculture, and had recently been granted honorary Afghan citizenship for decades of humanitarian work in the country's east. The gunmen fled the scene and police launched a search operation to arrest them, Sohrab Qaderi, a member of the governing council in the province of Nangarhar, told Reuters. Mr Qaderi believed Dr Nakamura had become a target because of his work. "Dr Nakamura has been doing great work in the reconstruction of Afghanistan," he said. No group claimed responsibility for the attack, but Zabihullah Mujahid, a spokesman for the Taliban, said the militant group was not involved. "The Afghan government strongly condemns the heinous and cowardly attack on Afghans' greatest friend, Dr Nakamura," said Sediq Sediqqi, a spokesman for Afghan President Ashraf Ghani. "He has dedicated all his life to change the lives of Afghans."