UN High Commissioner for Refugees Ruud Lubbers made the comment as relief efforts were stepped up to assist the huge number of refugees scattered around the country.
Mr Lubbers said that, although thousands of people in the north-west had begun to return home voluntarily, in some cities up to half the population had fled.
He welcomed the fact that more aid was beginning to arrive, but warned it was not enough to meet the desperate need.
"Much food and other things were brought in earlier, but it is still not at all a secure environment," he said.
Robbed
"Although our international staff went back to Kabul and hope to go back to the other places, one cannot say it is safe in the country and secure enough.
"So there is quite a job to be done in creating a secure environment. "
On Saturday, a convoy of supplies from the World Food Programme (WFP) arrived in Kabul - but only after the unarmed drivers had been robbed of their personal possessions along the route.
The convoy of 47 lorries was ambushed by bandits during its journey from Peshawar in northern Pakistan.
Protection plea
The WFP says it needs to deliver about 52,000 tonnes of supplies each month if it is to reach the most needy.
Calls for a UN-backed multi-national force to protect aid convoys have so far received a cool response.
On Thursday, the UN agency's trucks were robbed near Sarobi.
And UN premises in Jalalabad and Mazar-e-Shariff were also looted last week.
Meanwhile, in northern Afghanistan, a convoy from the International Committee for the Red Cross (ICRC) completed the four-day journey from neighbouring Turkmenistan to the city of Mazar-e-Sharif.
The 59 trucks took 1,500 tonnes of aid for distribution to 65,000 people in and around the city.
This included food and blankets plus medical supplies for hospital treating the war wounded.
First airlift
On Friday, the WFP carried out its first airlift of food aid to Afghanistan since 11 September.
The agency said it would fly four times a day from neighbouring Tajikistan, carrying food aid to the mountainous north-east of Afghanistan to feed those stranded in isolated villages.
The deputy UN coordinator for Afghanistan, Antonio Donini, said the UN was trying to expand the presence of its international staff, but was first checking security had been restored.
The UN says it is carrying out security assessments in Kabul, Faizabad and Mazar-e-Sharif, with one due to take place in Herat shortly.
Race against time
Mr Donini said the overall situation in Mazar-e-Sharif was still fragile, but there was a sense that it was improving.
He described the humanitarian situation in northern Afghanistan as a crisis of stunning proportions.
It was a race against time and said all corridors for delivering aid would be used as they became available.