Gunmen killed three health workers taking part in a polio vaccination drive in Karachi on Monday.

Gunmen killed three health workers taking part in a polio vaccination drive in Karachi on Monday, police said, in the latest blow to efforts to stamp out the disease in Pakistan.

Attackers on motorbikes opened fire on polio teams in two separate incidents in the Qayyumabad neighborhood in the east of Pakistan's biggest city.

The attacks came just days after the World Health Organization warned that Pakistan's northwestern city of Peshawar was the world's "largest reservoir" of the polio virus.

Immediately after the attack, the campaign was suspended in the east of the city and heavy contingents of police were deployed.

The vaccination campaign was suspended in parts of the city as a result of Monday's attacks, which police said also injured two people.

Militant groups see vaccination campaigns as a cover for espionage, and there are also long-running rumors about polio drops causing infertility.

According to the World Health Organization, Pakistan recorded 91 cases of polio last year compared with 58 in 2012.