UN aid officials also warn that access for humanitarian deliveries to Yemen remains a concern as the country's health care system is on the brink of collapse.
More than three-quarters of Yemenis are now in need of humanitarian aid as the war between Houthi rebels and the Saudi-backed government nears its fourth year, the United Nations said on Tuesday.
Some 8.4 million people are at risk of famine, up from 6.8 million in 2017, the UN humanitarian affairs office (OCHA) said.
A total of 22.2 million people, or 76 percent of Yemen's population of 29 million, are dependent on some form of assistance, an increase of 1.5 million people over the past six months.
The United Nations' efforts to address what it has described as the world's worst humanitarian crisis have been hampered by a crippling blockade of rebel-held ports by the Saudi-led coalition that intervened in support of the beleaguered government in March 2015.
Following a rebel missile attack on Riyadh airport in November, the coalition halted even UN aid deliveries but it has since eased that measure.
On Monday, a ship carrying four US-purchased mobile cranes that had been blocked by the coalition for months docked in the main rebel-held port of Hodeida.
Around 70 percent of Yemen's imports pass through Hodeida and UN spokesman Stephane Dujarric said the cranes will "significantly boost the discharge of humanitarian cargo."
"This will allow for faster delivery of relief items for Yemeni families in the grips of the world's biggest hunger crisis," he said.
Healthcare system near collapse
But UN aid officials warn that access for humanitarian deliveries remains a concern even if the blockade is lifted, as the country's healthcare system is on the brink of collapse.
"We appeal to parties on ground in order to stave off famine that we can continue regularly to get food, medicines in, be it from humanitarian or the commercial side," Bettina Luescher of the UN's World Food Programme (WFP) told a Geneva briefing.
Fadela Chaib of the World Health Organization (WHO) said that a diphtheria vaccination campaign had begun in the country, but the outbreak is "spreading quickly" with 678 cases and 38 associated deaths in four months.
Meanwhile, more than one million people have been infected with cholera, of whom more than 2,000 died, according to the International Committee of the Red Cross.
War has killed or injured 5,000 children
The war in Yemen has killed or injured more than 5,000 children and left another 400,000 severely malnourished and fighting for their lives, the UN children's agency said on Tuesday.
In a report unveiled in Sanaa, UNICEF said nearly two million Yemeni children were out of school, a quarter of them since the conflict escalated when a Saudi-led coalition intervened in March 2015.
More than three million children were born into the war, it said, adding they had been "scarred by years of violence, displacement, disease, poverty, undernutrition and a lack of access to basic services".
UNICEF said the more than 5,000 children killed or injured in the violence amounted to "an average of five children every day since March 2015".
"An entire generation of children in Yemen is growing up knowing nothing but violence," said Meritxell Relano, UNICEF representative in Yemen.
"Children in Yemen are suffering the devastating consequences of a war that is not of their making," he said in a statement.
"Malnutrition and disease are rampant as basic services collapse," he said, adding: "Those who survive are likely to carry the physical and psychological scars of conflict for the rest of their lives."
The UN agency said more than 11 million children - or "nearly every child in Yemen" - was now in need of humanitarian assistance.
It called for an end to the bloodshed and the protection of children, as well as sustainable and unconditional access to deliver assistance to every child in need.
A total of 9,245 people have been killed in Yemen since the coalition intervened in 2015, according to the WHO.
More than 50,000 have been wounded and millions displaced from their homes.