Mark Lowcock, the UN under-secretary general for humanitarian affairs, urged the Saudi-led coalition to lift its blockade of the conflict-torn country.
On Monday, the coalition shut air, land and sea routes into Yemen after Houthi rebels fired a missile at Riyadh.
The ballistic warhead was intercepted near the Saudi capital.
Saudi Arabia said the blockade was needed to stop Iran sending weapons to the rebels.
Iran denies arming the rebels, who have fought the Saudi-led coalition since 2015.