The New York-based IRC said it had "suspended its humanitarian aid programmes in Afghanistan indefinitely" following the attack.

"We are stunned and profoundly saddened by this tragic loss," George Rupp, the IRC president, said in a statement.

"These extraordinary individuals were deeply committed to aiding the people of Afghanistan, especially the children who have seen so much strife."

All four victims suffered multiple bullet wounds, a doctor said.

'Insulting Islam'

Abdel Latif, the provincial deputy police chief, said the attackers were disrespectful to Islam and Afghan culture.

"They are not supposed to kill women and these women were here to help educate our children," he said. "They were not politicians."

Zeina Khodr, Al Jazeera's correspondent in Logar, said the three were a Canadian, an American and an Irish.

She said Logar is one of the most dangerous provinces in Afghanistan even though it is approximately only 40km outside the capital of Kabul and it has been witnessing increasing attacks on aid workers and aid agencies.

In fact, Logar is considered to be a Taliban stronghold at the gates of Kabul, she said.

Increasing violence

Our correspondent also said senior UN sources had told her that more attacks like this one could see international non-governmental organisations (NGOs) leave the country.

An increase in violence has forced aid agencies to restrict their humanitarian and development work at a time when drought and high prices are putting more people under pressure, a group of 100 NGOs in Afghanistan said this month.

Many schools and clinics have closed and significant numbers of people have become internally displaced.

Aid workers said in a report this month that 19 aid workers had been killed in Afghanistan this year with spreading insecurity affecting relief work.

"Aid organisations and their staff have been subject to increasing attacks, threats and intimidation, by both insurgent and criminal groups," the Agency Co-ordinating Body for Afghan Relief said in a statement issued on August 1.

"This year there have been over 84 such incidents, including 21 in June, more than in any other month in the last six years.

"So far this year 19 NGO staff have been killed, which already exceeds the total number of NGO workers killed last year," the statement said.

Violence in Afghanistan has risen to its worst level since US-led multinational forces and Afghan forces overthrew the Taliban in 2001.

More violent incidents have occurred in each of the last three months than in any other month since 2001, aid agencies say.