At least 462,000 children suffering from severe acute malnutrition with 2.2m in need of urgent care, UNICEF says.
King Salman of Saudi Arabia has warned against external “interference in the internal affairs” of neighbouring war-torn Yemen.
The monarch also said on Wednesday his country would never accept that Yemen “becomes a base or a point of passage for whatever state or party to menace the security or the stability of the kingdom and of the region”.
Salman did not explicitly refer to the kingdom’s regional rival Iran, but Saudi officials have accused Tehran and the Lebanese Shia armed group Hezbollah of aiding Houthi rebels in Yemen.
International investigators last month said they had found a suspected “weapon pipeline” from Iran through Somalia to Yemen.
British-based Conflict Armament Research, which is primarily funded by the European Union, analysed photographs of weapons including assault rifles and rocket launchers to draw its conclusions.
Iran has repeatedly denied sending arms to Yemeni rebels.
Since March 2015, Saudi Arabia has itself led an Arab coalition conducting air strikes against the Shia rebels and providing other assistance to local forces in support of President Abd-Rabbu Mansour Hadi.
The rebels have killed at least 110 civilians and soldiers in rocket fire and skirmishes along the Saudi frontier. They have also fired longer-range ballistic missiles over the border at Saudi Arabia.
The Arab coalition, for its part, has faced repeated allegations of killing civilians. On Tuesday the United States blocked the transfer of precision-guided bombs to Saudi Arabia.
Salman’s son, Deputy Crown Prince Mohammed bin Salman, 31, has had overall responsibility for the Yemen campaign as he holds the post of defence minister.
The Yemen intervention has cost Saudi Arabia billions of dollars despite austerity at home to cope with fallen oil revenues.