Gunmen on motorcycles shot and killed seven police officers protecting polio vaccine workers Wednesday in the port city of Karachi, police said, the latest attacks in a nation where many fear aid groups harbor ulterior motives.

No health workers were harmed in either of the two attacks in Karachi, the capital of southern Sindh province, local police official Mohammad Ijaz said.

No group immediately claimed responsibility. The Taliban and other armed extremists have claimed the West is using vaccination programs as a cover for sterilizing or infecting children.

The distrust grew after investigators revealed a Pakistani doctor posed as a vaccination manager to obtain DNA samples of Usama Bin Laden's family, helping the CIA zero in on the terror leader's Abbottabad compound. In 2014, Pakistan gave Dr. Shakil Afridi a reduced prison sentence of 23 years after previously convicting him of treason.

Pakistan is one of just two countries that have never stopped the spread of polio, according to the World Health Organization. Scientists say the disease can paralyze and kill.

Officials had no plans to suspend the polio campaign despite the new attacks, senior police officer Feroze Shah said.

Earlier, provincial Home Minister Suhail Anwar told the Pakistani Geo news network that the seven officers were killed minutes apart. He said the attackers targeted police who had been deployed in the city in connection with the polio campaign, which was launched to vaccinate children.

Pakistani Prime Minister Nawaz Sharif strongly condemned the "terrorist attack" on police in Karachi. In a statement, he said police officers sacrificed their lives to secure the future of the coming generation.

The Associated Press contributed to this report.