Jan Vandemoortele, UN humanitarian coordinator in Pakistan, also urged India and Pakistan to open the Kashmir border, saying this would help the relief effort - if not solve logistical challenges posed by formidable Himalayan terrain.
Indian officials were set to arrive in Pakistan late on Friday for talks on letting Kashmiris cross the so-called Line of Control - a particularly sensitive issue for New Delhi because of a 16-year Muslim uprising in India's part of Kashmir by fighters seeking the territory's independence or merger with Pakistan.
The 8 October quake is believed to have killed nearly 80,000 people, the vast majority of them in northwestern Pakistan and Pakistan's portion of divided Kashmir.
"The situation is quite grim. With the money we have already, and much of it obtained from our own internal emergency reserves, we can keep the helicopters running for one week," Michael Jones of the UN World Food Programme (WFP) said in Islamabad.
The UN refugee agency also warned that its own reserves of emergency supplies were dangerously low.
With landslides still blocking many roads, helicopters are a lifeline for isolated communities, delivering supplies and ferrying badly injured people to hospitals.
Halting flights would be calamitous for hundreds of communities that have received little aid, weeks before the frigid Himalayan winter hits.
"The situation is quite grim. With the money we have already, and much of it obtained from our own internal emergency reserves, we can keep the helicopters running for one week"
UN World Food Programme
Donor nations meeting in Geneva this week pledged $580 million for quake victims, but much of it has not arrived.
The UN said it had so far received only about 20% of the funds needed for its emergency relief effort - a far weaker response than to other recent disasters, such as last year's Indian Ocean tsunami.
Jones, the WFP earthquake emergency relief coordinator, said an estimated 2.3 million people needed food. With its current funds, the agency could help just 500,000 people for two months, he said.
Suffering from disease
Thousands of quake survivors are still turning up each day at makeshift clinics, suffering increasingly from disease such as scabies, diarrhoea and pneumonia.
More than 3400 people sought treatment around the devastated northern town of Balakot on Thursday - 400 with suspected cases of acute respiratory infection - said Sacha Bootsma, communications officer for the World Health Organisation in Islamabad.
An estimated 2.3 million people
need food in northern Pakistan
"People are coming from remote areas to be treated," she said. "Increasing numbers are being treated for ARI (acute respiratory infection) because of the cold weather and lack of shelter."
On Friday, President General Pervez Musharraf approved two billion Pakistani rupees ($33.3 million) for reconstruction of homes, and said tents should be provided to the estimated 800,000 people without shelter within two weeks, according to his spokesman, Major-General Shaukat Sultan.
In a goodwill gesture, India on Wednesday offered $25 million to the UN quake appeal.
Indian and Pakistani officials were due to meet on Saturday in Islamabad on opening the Line of Control to let Kashmiris cross to help each other rebuild.
Vandemoortele said it would help the relief effort if aid workers could access Pakistani Kashmir from the Indian side.
India's military has started work on relief camps along the frontier and says that Pakistan could get building materials like cement and steel from Indian companies.
Musharraf has approved $33.3
million for reconstruction
However, both sides are yet to work out how to open the frontier amid lingering mutual suspicions about the other's intentions.
A Pakistani Foreign Ministry statement on Thursday noted that Pakistani quake victims "do not have to cross the Line of Control to find relief assistance".
Major Farooq Nasir, an army spokesman in Muzaffarabad - the capital of Pakistani Kashmir - also said on Friday that Pakistan has set up two more relief camps near the border.
The official death toll from the quake in Pakistan rose Friday to 56,000, but central government figures have consistently lagged behind those of local officials, which put Pakistan's toll at about 78,000. Another 1350 people died in Indian-held Kashmir.
US Ambassador Ryan Crocker pledged long-term American support on a visit to quake-savaged Muzaffarabad on Friday.
"This is a long-term disaster because of the terrain it's in and the approach of winter. We are going to be here for a while," he said on Friday at a US Army field hospital that started treating quake victims in the city this week.