"Seven million Yemenis do not know where their next meal will come from and are ever closer to starvation" in a country of 27 million people, Jamie McGoldrick said.
"Over 17 million people are currently unable to adequately feed themselves and are frequently forced to skip meals -- women and girls eat the least and last," he said in a statement.
Yemen's war pits the internationally recognised government of President Abedrabbo Mansour Hadi against Huthi rebels allied with forces loyal to former president Ali Abdullah Saleh.
The fighting has intensified since a Saudi-led coalition intervened in support of the government in March 2015 after the Huthis seized the capital the previous September.
Since early January, pro-government forces have pressed a major offensive aimed at recapturing Yemen's Red Sea coastline, and retook the southwestern port of Mokha earlier this month.
"I am deeply concerned with the escalation of conflict and militarisation of Yemen's western coast. It is coming at a great cost to civilians," McGoldrick said.
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Unexploded rockets have landed inside the rebel-held port of Hodeida, he said, "reducing even further the number of ships and imports" vital for Yemen's food supplies.
"Given that the country is 80-90 percent dependent on imported food staples, I am compelled to raise the alarm," the UN official said.
"If left unabated, these factors combined could accelerate the onset of famine."
Also on Tuesday, the UN children's agency warned that 462,000 children were suffering from acute malnutrition.
The UN aid chief warned last month that the impoverished Arabian Peninsula country was sliding deeper into humanitarian crisis and could face famine this year.
Stephen O'Brien said that without "immediate action", famine was "a possible scenario for 2017".
More than 7,400 people have been killed since the intervention began nearly two years ago, including around 1,400 children, according to the United Nations.