Fighting and airstrikes by a Saudi-led coalition "have impacted virtually the entire country," UN humanitarian coordinator for Yemen Johannes Van Der Klaauw said in a statement.
"The toll on civilians has been immense," he added.
At least 1,080 people had been killed in the country, including 48 children and 28 women, and a further 4,352 people had been injured in the violence between March 19 and April 20, according to the latest toll from the World Health Organisation.
The latest toll was published as Saudi-led coalition warplanes launched new strikes on Yemen despite a demand by Iran-backed Huthi rebels for a complete halt to the raids as a condition for UN-sponsored peace talks.
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WHO said the fighting so far had proved deadliest in the region around the capital Sanaa, where 209 people, including 21 children and five women had been killed.
Another 936 people had been injured there.
In the main southern city of Aden, 191 people have been killed, including two children, and another 1,237 injured.
He warned that the violence had "worsened an already large-scale humanitarian crisis" in the country, where more than 300,000 people were already internally displaced before the latest round of fighting erupted.
WHO warned earlier this week that the country's health system was on the verge of collapse amid shortages of medical supplies and fuel for generators.
Already, "cases of bloody diarrhoea, measles and suspected malaria have increased," Van Der Klaauw said.
The conflict has also disrupted the supply of food, fuel, water and electricity across Yemen, he said, lamenting that schools, health facilities and private homes had been damaged and destroyed and that some two million children were out of school.