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 19th and April 20th, according to the latest toll from the Geneva-based World Health Organization.
WHO receives its statistics from health facilities in Yemen, but since many people are not making it to hospitals for treatment the real numbers are probably higher.
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.
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.
More than 150,000 people have also been displaced inside Yemen by the conflict, Van Der Klaauw said.
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.
To help address the towering needs, the UN, which evacuated its international staff from Yemen at the end of March, was "exploring ways of redeploying (them) in the coming days," he said.
On Frida, the UN's children's agency said at least 115 children have been killed and 172 maimed in the violence raging in Yemen since Saudi-led air strikes began.
"We believe that these are conservative figures," Unicef spokesman Christophe Boulierac told reporters in Geneva, saying at least 64 of the children killed were victims of air strikes.