Elias Durry, head of WHO's polio eradication initiative in Pakistan, said the situation in Karachi was alarming.
Durre Naz Jamal, the deputy project director at the Sindh Expanded Programme on Immunisation, agreed with Durry and said, "The fact is that in many areas of the city anti-polio vaccination teams have been attacked or parents refused to have polio drops administrated."
Earlier on Tuesday, the posthumous diagnosis of the polio virus in a four-month-old baby girl from Baldia Town in Karachi had raised concerns about the efficiency of the government-run polio eradication campaign.
Pakistan is one of only three countries where polio is endemic. Immunisation campaigns have been affected by a ban imposed by the Taliban, who have killed several vaccinators in attacks in Karachi and the restive northwest.