KABUL, June 15 (Reuters) - Four polio vaccination workers were killed and three injured in separate attacks in Afghanistan"s eastern city of Jalalabad, a provincial health department official said on Tuesday, the latest in a series of attacks against health workers. A wave of assassinations have hit urban centres since peace talks began between the Taliban and the Afghan government last year in Doha, many of them targeting government employees, health workers, media and civil society members. Dr Jan Mohammad, the head of polio immunisation drive in Nangarhar of which Jalalabad is the main city, said gunmen targeted polio workers in three locations in the city that killed four and wounded three others. "Today was the second day of our operations after three months but we have to suspend it once again," Mohammad told Reuters, adding that all those killed were men. Gunmen killed three female polio vaccination workers in Jalalabad in March, that forced the workers to suspend their operations and assess security. No group claimed responsibility for Tuesday"s attacks but the Taliban insurgents fighting to overthrow the foreign-backed Afghan government, denied involvement in previous attacks. Zia ul Haq Amarkhil, the governor of Nangarhar said police were investigating the attacks. Many in Afghanistan"s conservative society oppose vaccinations, with militants frequently attacking health workers claiming they are being used by the West as a cover for spying. Afghanistan and Pakistan are the only countries in the world where polio remains endemic. Violence has sharply increased across Afghanistan since the United States announced plans in April to pull out all of its troops by Sept. 11.