All four victims suffered multiple bullet wounds, Dr. Mir Mabub Shah said as three female Afghan nurses shrouded the three dead women in white cloth before putting them in wooden coffins.

Zeina Khodr, Al Jazeera's correspondent in Logar, said the local authorities have determined the nationalities of the three foreign female aid workers killed in Afghanistan.

"A Canadian, American and Irish women were killed along with an Afghan staff member," Khodr said.

"The vehicle they were driving in came under attack by gunmen who shot and killed them.

"Logar is one of the most dangerous provinces in Afghanistan even though it is approximately only 40 kilometres outside the capital of Kabul.

"We have been witnessing increasing attacks on aid workers and aid agencies.

"A number of aid agencies have issued warnings that they are unable to operate in more and more areas of the country because they are now deemed unsafe.

"Last year two aid workers from the International Rescue Committee (IRC) were also killed on the same site," Khodr said.

Khodr said that senior UN sources had told her that more attacks like this one could see international Non-Governmental Organisations (NGOs) leave the country.

Increase in violence

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 non-governmental organisations 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. 

Khodr said the attacks are no longer confined to southern provinces it is spreading to others areas as well.

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

"More and more areas are becoming unsafe and it is not clear how the latest attack will affect the workers aid agencies across the country ... especially if these attacks continue and the government forces are unable to secure the roads reaching those areas affected by drought," Khodr 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.