One of the few remaining aid agencies in Iraq is pulling out of the country.
World Vision has been trying to improve the lot of children in Mosul
World Vision announced it was ending operations, following the murder of its senior manager in Iraq, and attacks on other aid workers.
Agencies like Care International and Medecins Sans Frontieres have already quit Iraq, saying it is too dangerous.
Meanwhile, the Red Cross has said its aid teams will not enter the city of Falluja without receiving assurances of their safety from insurgents there.
The United Nations refugee agency has said that tens of thousands of people are thought to have been displaced by the fighting in Falluja.
A UNHCR official said the agency had no staff on the ground and was extremely concerned about the displaced population.
World Vision has been in Iraq for 18 months, and says it has helped about 600,000 people, by improving schools, hospitals, clinics, and water supplies.
Its Iraqi head of operations, Mohammed
Hushiar, was shot dead by unknown gunmen in a crowded cafe in the northern city of Mosul on 29 September.
Tim Costello, chief executive of World Vision Australia, told the BBC World Service's World Today programme that maintaining security was becoming too much of a problem for aid agencies.
"We have realised that you can't have 24-hour security guards, and even now humanitarian agencies like ours, like Care - even the Red Cross with its studied neutrality for 150 years - are being targeted," he said.
"Your first priority is always your own staff and it has just become intolerable and too dangerous."
Mr Costello said Iraq represented "a whole new face of danger and violence" adding that the death of Care International's local director, Margaret Hassan, was "obscene and tragic".
Care ceased operations Mrs Hassan was kidnapped. She was apparently murdered weeks later.
World Vision has had 15-20 staff in Iraq, working mostly in Mosul and northern Iraq.
'Out of reach'
The International Committee of the Red Cross (ICRC) said on Friday that its aid teams could not enter Falluja without contacting insurgents fighting US-led
forces for control of the city.
"To enter the city we must be sure that all the warring parties accept our presence," ICRC spokeswoman Antonella Notari told AFP news agency.
"But one party to conflict is out of our reach - we cannot establish contact."
The ICRC had no information from the US military on when it would
allow aid teams into Falluja, she added.
The ICRC is providing aid
including tents, blankets, water and food to displaced people just outside
There has been no official count of civilian casualties during the fighting. US
forces said about 1,200 insurgents had been killed, while 51 US
soldiers and eight Iraqi government soldiers had also died.
The ICRC pulled its international staff out of most of Iraq and
scaled back operations after a car bomb exploded outside its
headquarters in the capital Baghdad in October 2003, killing 12 people.