Tim Costello, the CEO of World Vision, an Australian charity, told Al Jazeera there were already signs that diseases such as dysentery, diarrhoea and dengue fever were starting to take hold.
Eleven days after Cyclone Nargis left tens of thousands dead in Myanmar, the United Nations warned that another major storm could be heading for the area.
The UN weather centre is tracking a tropical storm off the coast of Thailand, and has said it could become "a significant tropical cyclone within the next 24 hours".
The ICRC said it arrived at the latest toll by pooling and extrapolating assessments by 22 other aid groups and organisations in 58 townships.
The total affected population is estimated to be between 1.6 million and 2.5 million, a statement said.
John Holmes, the UN humanitarian chief, also said that the toll could be "in the region of 100,000 or even more".
On Tuesday, the UN's emergency relief arm warned that Myanmar will "face a catastrophe of monumental proportions" unless aid efforts match the scale deployed during the Asian tsunami.
Holmes said: "The relief getting through, under the kind of restrictions we're operating under, is by no means adequate.
"So what we do need is a more multilateral, multinational response ... to help avoid that second wave of deaths that we're all fearing from the spread of disease among people weakened by the condition they're in."
Aid groups say the first 10 days are critical in getting relief to survivors of any disaster, but almost two weeks on that window has now closed for people in Myanmar.
With the toll steadily rising there are reports of soldiers blocking foreign aid workers from reaching some of the worst-hit areas.
The military has taken control of foreign aid supplies, and there are suspicions that relief supplies are being misappropriated after victims reported they are receiving poor quality food.
The warning of a second cyclone comes after a major aid group told Al Jazeera that hundreds of thousands of survivors from Nargis are facing a potentially "apocalyptic" threat from water-borne diseases.
Speaking from Yangon, he said the group's relief workers in the Irrawaddy delta were "gravely worried that we are tottering on the edge of this epidemic breaking out".
"If it does, its lethal effects will be apocalyptic," he said.
The bulk of the aid supplies that have been cleared to land in Myanmar are reported to be stuck on planes at Yangon airport, which does not have the lifting equipment needed to unload it.
As a result, the few aid agencies allowed to operate in Myanmar say they have reached only a third of the one million people left homeless by the cyclone.
The military government continues to refuse the assistance of foreign aid experts, who have vast experience in handling humanitarian crises.
It has granted visas to only 40 UN staff so far.
The concept of "self-reliance" is an idea propagated by the government, even during the devastation in Myanmar - an idea analysts claim is stemming from its deep suspicion of foreigners who have frequently criticised its human rights record.
Aid group approval
But General Thein Sein, the prime minister of Myanmar, told Samak Sundaravej, the Thai prime minister, on Wednesday that the government is in control of the relief operations and does not need foreign experts.
He said: "They have their own team to cope with the situation."
Samak also said that the government gave him a "guarantee" that there was no starvation or disease outbreaks among survivors.
Also on Wednesday, a Thai medical team was approved by the government to visit the Irrawaddy delta, according to Thawat Sutharacha, a worker at Thailand's public health ministry.
The government separately announced it will allow 160 relief workers from India, China, Bangladesh, and Thailand, to come to Myanmar, but it is not clear if they include the Thai medics or whether they can travel to the delta.
Holmes called the move "one small sign of selective opening up to international staff".