"If we don't move in today, if we don't do something very quickly, Yemen will be facing starvation very, very soon", the World Food Programme's Regional Director Muhannad Hadi said.
He said Yemen is experiencing a tragedy of vast proportions where especially children are already dying at an alarming rate.
A United Nations appeal for $1.2 billion this year for Yemen, where Guterres said a child under the age of five dies of preventable causes every 10 minutes, is only 15 percent covered.
About 17 million people are food insecure, he said, "making this the world's largest hunger crisis".
In March 2015, a Saudi-led coalition launched a military campaign to help the internationally recognised government regain control of territory lost to the Houthi rebels.More news: Canada introduces legislation to legalise marijuana
The two-year conflict has so far killed more than 7,800 people, forced over 3 million people from their homes and left 18.8 million people - 70 per cent of the population - in need of humanitarian assistance.
This means that about only 30% of the supplies Yemen needs are getting into the country at all.
A donors' conference in Geneva aims to drum up billions to ease the humanitarian crisis in the war-torn Arab nation.
"The risk of starvation is looming across the country and could become a reality without a massive global effort to fully fund the aid effort", the aid group's humanitarian manager Meg Quartermaine said.
"Our humanitarian appeal for 2017 is $2.1 billion and only 15 percent has been met until the present moment", Guterres said in his opening remarks to the gathering in Geneva. "We're insisting the fact that the situation in Yemen is so dire, without people getting money through our pledging conference, without people getting that turned into cash, turned into food, turned into health support, water, sanitation, people will invariably die", he said. These commitments today are an encouraging sign in the face of the goal of having the necessary funding in Yemen, he said. The injured and patients suffering from chronic diseases are dying avoidable deaths, while the most vulnerable groups-children, pregnant women, and the elderly-are at heightened risk of disease.More news: Trudeau defends supply management, wants 'fact' based trade talk with US
Sherine El Taraboulsi-McCarthy, Yemen expert and research fellow at the Overseas Development Institute, said she was very disappointed at the lack of recognition for the role of Yemeni people and civil society at the Geneva conference.
Amnesty International has also warned that a major new offensive by the Saudi Arabia-led coalition is due to get under way soon, saying that the strategic western port city of Hodeidah could reportedly be targeted.
At the pledging conference aid agencies will warn again that help can not come quickly enough for Yemen's people, especially its children. The United States, a long-time friend of the Saudis, has provided weapons and intelligence to the Kingdom on the sly, and President Donald Trump has recently considered ramping up the U.S.'s involvement. We must act now to save lives.
"The likelihood of a major operation in Hodeidah" should once again urge the global community to stop sending arms to warring parties in Yemen, Amnesty said.More news: Holidays, burka bans, Brexit vote - UK parties set out election stalls