The stark warning from World Health Organisation, Unicef and World Food Programme came 10 days after the Saudi-led military coalition shut down Yemen's sea and air ports.
An air raid by the Saudi-led military coalition fighting against the Houthi movement in Yemen shut down the airport in the capital Sanaa on Tuesday, further isolating the country where millions are on the brink of famine, the state news agency SABA reported.
Two suicide bombers detonated a car packed with explosives in Yemen's southern city of Aden on Tuesday, killing at least six people, witnesses said.
Young and old, desk clerks and activists, Yemenis from all walks of life took to the streets Monday to protest a Saudi-led blockade that has left thousands struggling to survive.
Saudi Arabia has blamed the Iran-allied Houthis for firing a ballistic missile towards Riyadh airport on November 4. Two days later, the Saudi-led coalition responded by closing access to Yemeni ports.
The United Nations and Red Cross on Tuesday urged the Saudi-led coalition fighting in Yemen to re-open an aid lifeline to bring imported food and medicine into the country.
Relief supplies into Yemen have been blocked since the coalition sealed off the country's borders following a missile attack by Yemen's Houthi rebels that was intercepted near Riyadh airport.
This "could be considered as an act of war," Saudi Crown Prince Mohammed bin Salman said.
The move follows the interception of a missile fired towards Riyadh on Saturday which the coalition has described as a "dangerous escalation" by the Iran-allied Houthi militia which controls large parts of Yemen.