Polio aid workers were killed in Pakistan while they were on their way to dispense vaccines to children, local police said Wednesday, adding that three others were injured in the tragic incident.

According to NBC News, some gunmen fatally shot the polio aid workers in Pakistan while they were riding motorcycles. It can be noted that Islamist militants have strong opposition against the vaccination campaign since they believe that this act is sterilizing Muslims.

The polio aid workers killed in Pakistan story happened near Quetta in Baluchistan on the border part of Afghanistan.

"Four were killed, including three women," said Naseer Baloch, the head of Baluchistan's health service.

The attack made by the Islamists militants on Wednesday was't the first. In fact, a number of polio aid workers have already been killed since the militants found out that Pakistani doctor Shakil Afridi only ran a vaccination campaign to gather DNA samples from Osama bin Laden and his family.

Afraid was reportedly trying to provide evidence to the CIA that bin Laden is staying there.

Polio is a crippling disease of the nervous system caused by the poliovirus. It affects the muscles for breathing and causes paralysis of the upper and lower limbs.

In 2013, approximately 85 percent of polio cases were recorded in Pakistan. This year, the polio cases has increased four times that last year's figure, reports New York Times.  

Amid the prevalence of the disease in Pakistan, Prime Minister Nawaz Sharif and other politicians have promised to help eliminate the spread of polio in the country.

Nevertheless, preventing the Islamists militants from targeting polio aid workers has been a far-off dream. 

In the recent assassination in Quetta, local reports state that the gunmen opened fire on the polio aid workers vehicle after asking them whether they were involved in the anti-polio campaign. 

Television footage after the incident showed responding emergency workers carrying three badly wounded polio aid workers from the van.

Police said the two gunmen were armed and were riding a motorcycle when they killed the polio aid workers in Pakistan, reports Los Angeles Times.

The attackers are currently at large, and no one has claimed responsibility over the incident as of late.

"There was a policeman standing at a distance, but he didn't come to our aid. We called everyone but no one came to our rescue," one worker who survived the attack told reporters at the city's Civil Hospital.

Since July 2012, 57 polio aid workers have been killed in Pakistan.