"The devastating conflict in Yemen takes place against the backdrop of an existing humanitarian crisis that was already one of the largest and most complex in the world," a UN statement issued in Jordan said.
UN Humanitarian Coordinator Johannes Van Der Klaauw said fighting as well as air strikes by a Saudi-led coalition against Shiite rebels had forced thousands of families to flee their homes.
"Ordinary families are struggling to access health care, water, food and fuel -- basic requirements for their survival," he said.
The appeal addressed to the international community by the United Nations and its humanitarian partners aims "to meet the life-saving and protection needs of 7.5 million affected" by the conflict.
The Saudi-led coalition launched its air war on March 26 against the Shiite rebels known as Huthis who overran much of Yemen and forced President Abedrabbo Mansour Hadi to flee the country.
Signup to our newsletter and follow us on Facebook and Twitter!
"Air strikes have now affected 18 of Yemen's 22 governorates. In the south, conflict has continued to intensify, particularly in Aden, where widespread fighting continues," the UN said in its statement.
It noted that violence has spilled into residential neighbourhoods and that civilian and religious infrastructure -- including mosques, schools, hospitals and airports -- have been damaged.
Hundreds have been killed and nearly 3,000 wounded "and there are reports of serious violations of human rights and international humanitarian law," it said.
The UN hopes it will be able to raise the funds quickly to provide much-needed food, medical supplies, drinking water and emergency shelters.
It also insisted that "physical protection of civilians is a top priority".
The UN said access to people in need in Yemen was largely hampered by the violence shaking the country and logistical hurdles, citing "difficulties in bringing emergency supplies and aid workers".
"I urge donors to act now to support the people of Yemen at this time of greatest need," said Ven Der Klaauw.