The World Health Organisation on Thursday unveiled a 28 million dollar (18 million euro) "This is a very serious crisis," said Eric Laroche, the head of the WHO's Global Health cluster effort to respond to Myanmar who had just returned from the country.
Communicable diseases are the greatest worry with the malaria season approaching, as well as the risk of dengue fever and cholera, he told journalists.
Malnutrition is also a serious threat, leaving people more vulnerable to infections, Laroche added.
Nearly four weeks after Cyclone Nargis pummelled large swathes of Myanmar and left more than 133,000 people dead and missing, the UN estimates that foreign aid has still only reached 40 percent of the 2.4 million needy survivors.
Myanmar's isolated military had largely barred foreign aid workers from gaining access to the southwest Irrawaddy Delta, which bore the brunt of the cyclone, but Laroche said access is now improving.
"It is much easier to get visas," he said, adding that he had received his own within 48 hours.
Last Friday, UN Secretary General Ban Ki-moon said he had received assurances that Myanmar's military regime would grant access to all foreign relief workers.
Richard Horsey, spokesman for the UN's emergency relief arm, said the situation was "tentatively positive", with international UN staff able to move into the delta after giving the regime 48 hours' notice.
The WHO action plan aims to assess and monitor health needs and strengthen disease surveillance, as well as rebuild Myanmar's ravaged healthcare system.
The cyclone destroyed about 50 percent of the healthcare system in the affected region.
"A rapid assessment of the damage to health facilities is needed to understand what is required immediately," the WHO said.
"Additional supplies of essential emergency medicines, equipment, bed nets, and provision of mental health and psychological support for survivors suffering trauma can be foreseen," it added.