"We will not stand by and watch millions of already vulnerable men, women, and children become victims of an avoidable catastrophe", said Dr. Peter Salama, WHO Executive Director for Emergencies.
The Prime Minister vowed to continue support of the famine-hit country as she opened a major worldwide conference focusing on the future of the East African nation.
He said political stability has improved but the gains are fragile in part because "growing food insecurity" is affecting millions of Somalis.
May said Somalia had made progress since five years ago, when al Shabaab controlled large parts of it, piracy was costing global trade $7 billion a year and the country was recovering from a starvation that killed a quarter of a million people.
The president of Somalia, Mohamed Abdullahi Farmajo, recently declared war against al-Shabab and has given himself a deadline of two years to defeat the group. Aid agencies have expressed concern that the military moves could endanger the hundreds of thousands of people displaced by the drought.
He spoke to reporters at the end of a high-level conference to address the Horn of Africa nation's deepening humanitarian and security crisis.
More than 620,000 people have left their homes because of the drought since November, the United Nations says, slowing farmers' work to prepare their land and plant crops as the rainy season starts.
The group attacks government, military and civilian targets in the capital and elsewhere, often deploying suicide bombers, and has over-run several military outposts, massacring soldiers from the 22,000-member African Union Mission in Somalia, known as AMISOM.
Theresa May said the aim was "to degrade al-Shabaab by having Somali forces take over the security of the country".
Drought has led to the largest cholera outbreak in the last five years, with more than 36,000 cases and almost 700 deaths so far this year, the World Health Organization says.
"Restoring Somalia's relations with the World Bank, cancelling the country's debt, and providing immediate financial support from the World Bank's worldwide development association facility is critical", he said ahead of the meeting.
Save the Children CEO Kevin Watkins said: "Somalia continues to drift towards an avoidable starvation".
Starvation has already gripped South Sudan, with hunger killing many there and in Somalia, while a national emergency has been declared in Kenya and there are fears of a new wave of drought in Ethiopia.
Earlier in the day, the World Health Organization (WHO) called on the global community for immediate action to save lives in the crisis-torn country.