Armed men in Pakistan killed three health workers taking part in a polio-vaccination drive and wounded two others on Tuesday –– the latest in a series of deadly assaults on vaccination teams in the country.

Men on motorbikes opened fire on polio teams in two separate incidents in the Qayyumabad neighborhood of Sindh province in the east of Karachi, Pakistan's largest city. Those killed in the attacks were between 24 and 31 years old. Police said a passerby was also injured. No one immediately claimed responsibility for the attacks.

Seemi Jamali, head of the Jinnah Hospital, confirmed that three bodies and two wounded people were taken to her facility.

Tuesday's attacks came a day after Sindh's health authorities began to immunize up to 7.6 million children with polio vaccines as part of a three-day nationwide drive. But the polio workers' association of Sindh said it was now halting operations across the province.

"We will not carry out the campaign from now on," Khairun Nisa Memon, the head of the association, said outside Jinnah Hospital. "We have lost lives of our workers today and that will happen again and again."

Pakistan is one of only three countries in the world where polio remains endemic, along with Afghanistan and Nigeria. It is the only polio-endemic country where the number of cases rose from 2012 to 2013, from 58 to 91, according to the World Health Organization (WHO).

The WHO on Friday said that Pakistan's northwestern city of Peshawar was the world's "largest reservoir" of the poliovirus. All but one of Afghanistan's 13 cases last year could be traced back to poliovirus strands found in Peshawar.

Last year, more than 30 attacks on polio teams –– who only travel with police protection –– took place, killing more than 30 immunization workers since July 2012.

Armed anti-government groups in Pakistan see vaccination campaigns as a cover for espionage. Immunization programs, especially those with international links, have come under suspicion since a Pakistani doctor ran a fake vaccination scheme to help the CIA track down Osama bin Laden. Pakistani Taliban commanders since mid-2012 have forbidden polio teams to access some areas.

Saira Afzal Tarar, Pakistan's junior health minister, denounced the assault: "It was an appalling act. We strongly condemn it." She said the authorities were waiting for a report on the incidents.

"We will hold a meeting on this tomorrow," she said. "After that, we will direct the provincial governments on how to save the lives of those who are out to save lives of others."

Polio can permanently paralyze or kill victims within hours of infection. Intensive vaccination campaigns have almost eradicated the disease worldwide. In 1988, the disease was endemic in 125 countries and paralyzed roughly 1,000 children every day. The number of polio cases has decreased by more than 99 percent since then, but areas marked by severe violence remain out of reach.

Somalia and Syria have also witnessed increases in the number of polio infections due to increased violence toward health workers and war, respectively.

In contrast, Pakistan's neighbor India last week celebrated three years since its last polio case.

Al Jazeera and wire services