Armed men in Pakistan have killed three health workers taking part in a polio-vaccination drive - the latest in a series of deadly assaults on vaccination teams.

Following Tuesday's deaths in Karachi, the polio workers' association of Sindh said it was halting operations across the province.

Men on motorbikes opened fire on polio teams in two separate incidents in the Qayyumabad neighbourhood in the east of Karachi, Pakistan's biggest city.

Those killed in the attacks were between 24 and 31 years old, with no one immediately claiming responsibility for the attack.

Police said a passer-by was also injured.

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

Armed anti-government groups in Pakistan see vaccination campaigns as a cover for espionage, and there are also long-running rumours about polio drops causing infertility.

"The attackers wearing helmets were waiting for the teams on motorcycles," a police spokesman told AFP news agency.

Tuesday's attacks came a day after Sindh's health authorities began to immunise up to 7.6 million children with polio vaccines as part of a three-day nationwide drive.

Campaign halted

Khairun Nisa Memon, the head of the provincial polio workers' association, said vaccinations would stop as a result.

"We will not carry out the campaign from now on," she said outside Jinnah Hospital.

"We have lost lives of our workers today and that will happen again and again."

Saira Afzal Tarar, Pakistan's junior health minister, denounced the assault after inaugurating a laboratory in the Pakistani capital Islamabad.

"It was an appalling act. We strongly condemn it," she said.

Tarar 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."

Speaking to Al Jazeera, Moeed Yusuf of the United States Institute of Peace (USIP), Washington DC, said: "The real problem is that the state's capacity in the rural areas has shrunk to a point where it can simply cannot defend or protect its own citizens and communities."

The Karachi shootings come just days after the World Health Organisation (WHO) said that Pakistan's northwestern city of Peshawar was the world's "largest reservoir" of the polio virus.

According to WHO, Pakistan recorded 91 cases of polio last year compared with 58 in 2012.

By contrast, the country's neighbour India last week celebrated three years since its last polio case.

Repeated attacks

Polio can permanently paralyse or kill victims within hours of infection.

Intensive vaccination campaigns have almost eradicated the disease worldwide.

Pakistan is one of only three countries in the world where polio remains endemic, along with Afghanistan and Nigeria.

Efforts to eradicate it have been seriously hampered by the repeated targeting of vaccination teams in recent years.

Many teams only travel with police protection.

Last year there were more than 30 attacks on polio teams.

Pakistani Taliban commanders have forbidden vaccination teams to access some areas. A handful of religious leaders have also denounced the campaign as a plot to sterilise Muslim children.

"This is essentially political rhetoric mixed with religious edicts which absolutely makes no sense to a rational mind," Yusuf of USIP told Al Jazeera.

"There is a whole new political twist to this after the controversy around the raid that killed [al-Qaeda chief] Osama bin Laden [in the Pakistani city of Abbotabad] and this doctor who was apparently running a fake polio campaign being indicted."