LAHORE, Pakistan - Under police guard, thousands of health workers pressed on with a polio immunization program Thursday after nine were killed elsewhere in Pakistan by suspected militants opposed to the vaccination campaign.

Immunizations were halted in parts of Pakistan and the U.N. suspended its field participation everywhere until better security was arranged for its workers. The violence risks reversing recent progress fighting polio in Pakistan, one of three countries where the disease is endemic.

The Taliban has denied responsibility for the shootings. Militants have accused health workers of acting as spies for the U.S., alleging the vaccine is intended to make Muslim children sterile.

Taliban commanders in Pakistan's troubled northwest tribal region said this year that vaccinations can't go forward until the United States stops drone strikes in the country.

Insurgent opposition to the campaign grew last year after it was revealed that a Pakistani doctor ran a fake vaccination program to help the CIA track down and kill al-Qaeda founder Osama bin Laden, who was hiding in the town of Abbottabad.

There were a few attacks on polio workers in July, but the current level of violence is unprecedented. A polio worker died Thursday after being shot in the head in Peshawar a day earlier, said health official Janbaz Afridi. His death raised to nine the number of Pakistanis working on the campaign who have been killed this week. Two other workers were critically wounded.

Despite the threat, officials in the eastern city of Lahore continued the vaccination drive Thursday, said a government official there, Noorul Amin Mengal. He said about 6,000 Pakistani health workers were escorted by 3,000 police as they fanned out across the city.