Humanitarian organisations are taking advantage of a five-day truce to get aid into Yemen after weeks of Saudi-led air strikes targeting Houthi rebels.
UN agencies and their partners hope to replenish desperately needed supplies of food, water, medicine and fuel.
Ships bearing aid have docked at a Red Sea port and planes are standing by.
The ceasefire has broadly held since it began on Tuesday night, although intermittent clashes were reported across the country on Wednesday.
The Saudi-led coalition has said the pause in its seven-week-old air campaign is conditional on the rebels reciprocating and not exploiting it for military advantage.
Residents said there were clashes in the southern city of Aden, the nearby provinces of Daleh, Shabwa, Lahj and Abyan, as well as the third city of Taiz, after the ceasefire started at 23:00 (20:00 GMT) on Tuesday, but that they ended soon after midnight.
"Sanaa had a quiet night as the noise of bombs and anti-aircraft fire that had terrified everyone came to a halt," a resident of the capital, Tawfiq Abdul Wahhab, told the AFP news agency.
Later on Wednesday, the US state department said it had received reports of some clashes and anti-aircraft fire, but that it understood that the ceasefire had "broadly held".
The UN's emergency relief co-ordinator Valerie Amos called on all parties to the conflict to respect the "vital pause in hostilities", which she said would provide "respite for civilians".
There are currently 12 million people without access to sufficient food, clean water, fuel or basic medical care. As many as 300,000 have fled their homes.
Two cargo ships chartered by the UN's World Food Programme (WFP) are docked at the rebel-controlled Red Sea port of Hudaydah. Other supplies are ready to be brought in and planes are standing by to help evacuate the wounded.
The International Committee for the Red Cross (ICRC) said it was sending medical and surgical supplies for 700 patients by boat to Aden, which has seen some of the fiercest fighting. Medical supplies and water would be distributed to other provinces in the south if the situation allowed, it added.
However, the charity Oxfam has warned that five days is "not enough time to move supplies into and around the country, particularly with fuel supplies dangerously low".
Saudi Arabia's King Salman has meanwhile doubled the money pledged by the kingdom for the aid effort to $530m (£340m).
Baroness Amos requested that aid be routed through existing UN and international humanitarian organisation channels and stressed: "It is essential that humanitarian assistance is not politicised."
Her warning came as Iran's foreign ministry insisted it would not allow Saudi-led naval forces to inspect an aid ship bound for Hudaydah under military escort.
The US has urged Iran to redirect the ship - said to be carrying 2,500 tonnes of food, medicine, tents and blankets - to Djibouti, where the UN has set up a hub.
Saudi Arabia has accused Iran of providing military assistance to the Houthis, a charge it denies.
In the days leading up to the ceasefire, the coalition stepped up its air strikes in an apparent attempt to inflict as much damage as possible on the Houthis and allied security personnel loyal to the ousted former president, Ali Abdullah Saleh.
The UN says at least 828 civilians have been killed and 1,511 injured since the start of the coalition air campaign on 26 March to restore exiled President Abdrabbuh Mansour Hadi. The six days from 4 to 10 May were the deadliest, with at least 182 civilians reported killed.