Wolfgang Jamann, the head of the NGO, raised alarm over the country's deteriorating situation since March 2015 when Saudi-led forces launched a military operation in support of the government against the Iran-backed Houthi rebels.
"We are now in the 21st century and the current situation is an absolute shame on humanity," Jamann told reporters on Saturday after a five-day visit to the impoverished Arabian Peninsula country.
"Thousands of civilians have died since the start of the conflict and millions more have been displaced inside the country," he told a news conference.
Jamann said "60 percent of the country is food insecure and over half the population is unable to access safe drinking water."
"Many areas in Yemen are just one step away from a famine situation," he said, and urged the international community to "end the suffering".
More than 44,500 people are estimated to have been wounded.
The situation in the country of some 27 million has been worsened by a massive outbreak of the bacterial infection cholera.
More than 600,000 people are expected to contract cholera in Yemen this year, the International Committee of the Red Cross (ICRC) warned on Sunday.
One in every 45 Yemenis will have contracted the disease by December as "a direct consequence of a conflict that has devastated civilian infrastructure and brought the whole health system to its knees," the ICRC said in a statement.
More than 370,000 people have fallen ill and 1,800 have died since late April in Yemen's second cholera outbreak in less than a year, according to the ICRC and WHO.
According to the WHO, 5,000 new suspected cases of cholera are registered every day in Yemen, which is facing the world's largest outbreak of the disease.
A string of vital ports along the country's Red Sea coastline are blockaded, leaving millions of people with limited access to food and medicine.
Less than half of the country's medical facilities are currently functional.