Gunmen killed five Pakistani women working on a U.N.-backed polio vaccination campaign in two different cities on Tuesday, officials said. The attacks were likely an attempt by the Taliban to counter an initiative the militant group has opposed.

The attacks came a day after an unknown gunman killed a male volunteer for the World Health Organization's anti-polio campaign in Pakistan's largest city, Karachi.

The government is in the middle of a three-day vaccination campaign in the highest risk parts of the country, part of an effort to vaccinate millions of children under the age of five.

Four of the women killed Tuesday were gunned down in Karachi, said Sagheer Ahmed, the health minister for surrounding Sindh province. Two male workers were critically wounded in the shootings, said Ahmed, who wrongly reported earlier that one of them had died.

The attack on the polio workers was well-coordinated and occurred simultaneously in three different areas of the city, said police spokesman Imran Shoukat.

The government suspended the vaccination campaign in Karachi in the wake of the shootings, said Ahmed. The campaign started on Monday and was supposed to run until Wednesday, he said.

Gunmen on a motorcycle also shot to death a woman working on a government anti-polio campaign in a village near the northwestern city of Peshawar, said Janbaz Afridi, a senior health official in surrounding Khyber Pakhtunkhwa province.

He said the attacks would not stop the government from continuing its vaccination program in the province and the neighboring tribal region, the main sanctuary for Taliban militants in the country.

"These incidents are depressing and may cause difficulties in the anti-polio drive, but people should not lose heart," said Afridi. "The government is very serious, and we are determined to eliminate polio despite all odds and difficult conditions."

The Taliban have spoken out against polio vaccination in recent months, claiming the health workers are acting as spies for the U.S. and the vaccine itself causes harm. Militants in parts of Pakistan's tribal region have also said the vaccination campaign can't go forward until the U.S. stops drone attacks in the country.

The shootings in Karachi on Tuesday all took place in areas mainly populated by ethnic Pashtuns, said Ahmed, the health minister. The Taliban are a Pashtun-dominated movement, and many militants are reported to be hiding in these communities in Karachi.

The immunization campaign was suspended in Karachi in July as well after a local volunteer was shot to death and two U.N. staff were wounded.

Pakistan is one of only three countries where polio is endemic. The virus usually infects children living in unsanitary conditions, attacks the nerves and can kill or paralyze.

The government, teaming up with U.N. agencies, is on a nationwide campaign to give oral polio drops to 34 million children under the age of five.

But vaccination programs, especially those with international links, have come under suspicion in the country since a Pakistani doctor ran a fake vaccination program last year to help the CIA track down Osama bin Laden.

Also Tuesday, two men on a motorcycle hurled hand grenades at the main gate of an army recruiting center in northwestern Pakistan on Tuesday, wounding 10 people, police said.

The injured in the attack in the garrison town of Risalpur in Khyber Pakhtunkhwa included civilians and security personnel, senior police official Ghulam Mohammed told The Associated Press. The police have launched a manhunt to trace and arrest the attackers, he said.

No one immediately claimed responsibility for the attack, the latest in a string of assaults in recent days that illustrate the continued challenge Pakistan faces from militants despite military operations against the Pakistani Taliban and their supporters.

Tuesday's attack came a day after a car bomb exploded in a crowded market in Pakistan's northwestern town of Jamrud near the Afghan border, killing 17 people and wounding more than 40 others.

Khyber Pakhtunkhwa is located on the edge of Pakistan's tribal region, the main sanctuary for al-Qaida and Taliban in the country. The province has witnessed scores of attacks, most of them blamed on the Taliban.

Ten Taliban fighters armed with rockets and car bombs attacked the military section of an international airport in the northwestern city of Peshawar on Saturday night, killing four people and wounding over 40 others. Five of the militants were killed during the attack and the other five died Sunday after hours-long shootout with security forces.

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Associated Press writer Jamal Khan contributed to this report from Peshawar, Pakistan.