ISLAMABAD, Nov. 27 -- The World Health Organization ( WHO) on Thursday suspended operations in Pakistan's Balochistan province a day after unidentified gunmen shot dead four polio workers including three women, local media reported.

The gunmen sprayed bullets at a van, carrying a team of nine women polio workers, near Quetta, the provincial capital, on Wednesday.

No group has claimed responsibility for the attack that has further raised concerns about the safety of polio vaccination teams. The government blames militant groups for such attacks.

Dozens of polio volunteers and their guards have been killed in targeted attacks in parts of Pakistan in recent months.

The injured women polio workers had complained that they were not provided security.

Officials on Thursday suspended several police officers for failure to protect the polio workers.

The assailants fled after the attack and the police have not yet succeeded to arrest the suspects.

TV channels reported that the WHO decided to suspend its operations in Balochistan for an indefinite period over security concerns.

There was no official word from the WHO on the reports.

The WHO and the United Nations Children Fund have been assisting the provincial government of Balochistan in eradicating polio virus from the province for more than twenty years.

The media quoted an unnamed WHO official as saying that the organization has closed its offices in the province after the attack.

"We will not make programs to motivate the people to administer polio drops to their kids," Dawn TV quoted the official as saying.

Health officials say that 12 new polio cases have been found in Balochistan this year.

A polio vaccination campaign was launched amid tight security in 11 districts of Balochistan earlier this month. Balochistan had earlier been polio-free for almost more than two years. The first polio case was reported from Balochistan's Killa Abdullah district in July this year.

The lady health workers, who had been involved in the vaccination program, have also boycotted the campaign in Quetta.