Cyclone Nargis, which slammed into Myanmar's southern coast on Saturday, has left at least 22,000 people dead and another 41,000 missing by the official count, but the toll is expected to rise.
Kirkwood said the organisation's staff had gathered harrowing eyewitness accounts from the worst-hit area of the Irrawaddy Delta region, a low-lying agricultural region which was inundated by a huge storm surge.
"One team came across thousands of people killed in one township, with piles of rotting bodies lying on the ground as the water had receded," he said.
He said there were "really worrying" reports that people were dying in the town of Pyinkaya in the southwest of the delta, home to 150,000 people, which received no supplies of food or clean water since the storm hit.
"Assistance hasn't reached them yet and they are dying - completely isolated," he said.
Government red tape
Countries around the world are pledging aid in the wake of the devastation in Myanmar, but few relief agencies have actually been able to get on the ground.
Although the military government has given the green light to foreign aid in principle, many agencies have complained that red tape and the government's inexperience have delayed efforts.
There are also concerns over what the military government will do with the money.
The United Nations says a million people may have been displaced and the situation on the ground is desperate, with survivors sleeping where they can and running out of food and water.
The government has been criticised for responding slowly, before and after the cyclone struck.
Meteorologists say they gave 48 hours warning before the storm, but people were not told early enough to evacuate.
A spokesman for the UN's World Food Programme said on Tuesday it had begun distributing food in damaged areas of Yangon and that 800 tonnes of food had already arrived.
But many other aid providers were still waiting for visas to allow their teams entry into Myanmar, including a five-person UN disaster assessment team waiting to go in from Bangkok.
Two US navy ships are waiting near the waters of Myanmar for its green light to carry out evacuation and other critical relief activities.
Aid abuse fears
George Bush, the US president, said: "We're prepared to move US navy assets to help find those who have lost their lives, to help find the missing, to help stabilise the situation.
"But in order to do so, the military junta must allow our disaster assessment teams into the country."
|Logistical problems are also hampering aid to |
the isolated Irrawaddy delta [Reuters]
Some refugees from Myanmar have criticised Bush for setting conditions on aid in their country's hour of need.
But others are pushing for caution, saying they do not want humanitarian assistance to end up enriching the generals who run the country.
They have warned that aid may be funnelled off to the military if it is not distributed directly to the people who need it.
Bush Gulati, with the Committee for the Restoration of Democracy in Burma, said: "If any aid goes in the hands of the government, even if it's a blanket, the army will use it for themselves."
Countries such as France have also expressed concerns over how the relief will be distributed.
It seems clear the military rulers in the secretive country are trying to control the situation.
Rashid Khalikov, director of the Geneva-based UN Office for the Co-ordination of Humanitarian Affairs, said that efforts to assist hundreds of thousands of people in need of help were being hampered by the government's inexperience and red tape such as the need for visas.
The scale of the devastation is also posing logistical problems for relief teams, with communications and transport severely affected.
The worst-hit Irrawaddy delta remained largely cut off from the rest of the world four days after winds, floods and high tidal waves tore through the densely populated region.
Despite all that, governments and aid agencies have already pledged more than $10m in aid with the European Commission and the US contributing $3m each.
Walter Lohman, a South-East Asian expert at the Washington-based Heritage Foundation, said Myanmar's military rulers now face a difficult decision because of years of isolation from the rest of the world.
"The generals have a choice to make between helping the Burmese people - who are in dire need of help - or out of fear of losing their total grip on power and spoils, effectively turning away international assistance," he said.