PESHAWAR, Pakistan — Gunmen opened fire on a polio-vaccination team in Pakistan’s tribal belt Monday, killing a paramilitary soldier and underscoring the continued threat to one of the region’s most urgent health campaigns.

Gunmen hiding in a field fired at the health workers as they were traveling through Bajaur tribal district, which borders Afghanistan. A paramilitary soldier who had been guarding the team was killed, and the gunmen fled.

The health workers were part of a three-day drive to vaccinate children younger than 5 against polio in the tribal belt, which is the major center of new infections in Pakistan. The country is one of just three where the disease remains endemic.

Health officials say that 46 new cases of polio were detected in northwestern Pakistan in 2012, of which 20 came from the tribal belt. The Pakistani health authorities, with backing from Western donors and the United Nations, regularly conduct immunization drives that target affected children.

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The current three-day campaign involves 682 teams that aim to administer vaccination drops to 223,570 children.

A senior tribal official said that Monday’s attack was the first in that part of Bajaur, a mountainous district that has seen fierce fighting between the Pakistani army and Taliban fighters.

No group claimed responsibility for Monday’s shooting, but suspicion fell on the Pakistani Taliban, who have declared polio vaccination un-Islamic and have carried out previous attacks on polio workers. More than 20 Pakistanis have been killed in attacks on polio teams across the country in recent months.

However, a leader of the Afghan Taliban, a related but separate group, last week said it would facilitate polio vaccination in Afghanistan if United Nations teams used neutral health workers and respected Islamic traditions.

If the World Health Organization and UNICEF followed those directives, the statement said, then the Afghan Taliban were under orders “not to put hurdles in their way and rather offer them help.”

A senior tribal official in Peshawar said the polio campaign would continue despite the attacks.