Eight members of polio vaccination teams were killed in separate attacks in Afghanistan, the United Nations said on February 24.
"We are appalled by the brutality of these killings, across four separate locations," the UN said in a statement.
"This senseless violence must stop immediately, and those responsible must be investigated and brought to justice. These attacks are a violation of international humanitarian law,” the statement added.
The UN said one person was killed in Takhar Province in the far north, and seven in neighboring Kunduz Province -- including four in the provincial capital, Kunduz city.
They were engaged in house-to-house visits or on their way to begin campaigns, the UN said.
Obaidullah Abidi, a spokesman for the Taliban police in Kunduz, told Radio Azadi that four women were among those killed.
Matiullah Rohani, head of the Taliban's Information and Culture Department in Kunduz, said security forces are working to arrest the perpetrators.
"Our security and intelligence agencies have launched an operation in connection with the incidents that took place today in Kunduz Province, and efforts are under way to apprehend the perpetrators,” Rohani told Radio Azadi.
The killings were the first since UNICEF and the World Health Organization launched a nationwide polio vaccine campaign in November aimed at reaching over 3 million children, with the backing of the Taliban.
Polio teams were frequently targeted by insurgent groups in Afghanistan until the Taliban's takeover of the country last year, when the hard-line Islamist group said it wanted to work with the UN to stamp out the disease.
Last year, several polio vaccinators were shot by unknown militants in eastern Nangarhar province.
No group has claimed responsibility for the recent attacks.
Afghanistan and Pakistan are the only two countries in the world where polio has not been eradicated, although Pakistan last month reported a year without a case for the first time in history.