Aid agencies are attempting to reach areas of Yemen hardest hit by seven weeks of air strikes and fighting, as a humanitarian ceasefire largely holds.
Fuel, food and medicine are being distributed at the port of Hudaydah and transported to neighbouring provinces.
The UN says hundreds of thousands of people are in desperate need of help.
Houthi rebels have been accused by the Saudi-led coalition seeking to restore the exiled president of violating the five-day truce that began on Tuesday.
On Wednesday, the coalition said rockets fired from rebel-held northern Yemen had hit the Saudi border provinces of Jazan and Najran, but that Saudi forces had "practised self-restraint as part of their commitment" to the ceasefire.
Coalition warships were meanwhile reported to have shelled rebels trying to seize an area outside the southern city of Aden, and warplanes bombed an alleged rebel convoy in the northern province of Abyan.
There was also heavy fighting in the southern province of Taiz between rebels and militiamen loyal to the internationally-recognised president, Abdrabbuh Mansour Hadi, who fled abroad in March.
'Provocations'
After meeting political leaders in the capital Sanaa on Wednesday, the new UN special envoy to Yemen noted "with concern" the continued fighting in some areas.
Ismail Ould Cheikh Ahmed called on all parties to "resist reacting to provocations aimed at undermining the truce which could lead to a resumption of violence".
"Further violence could hinder the provision of humanitarian aid and relief of the Yemeni people and undermine prospects for a permanent ceasefire and a return to the political process," he warned.
The UN Office for the Co-ordination of Humanitarian Affairs (OCHA) said partner organisations had used the first 24 hours of the humanitarian pause to "re-supply, pre-position stocks and carry out response".
Mobile medical teams travelled to affected areas in Hudaydah, Amran and Hajjah provinces.
Yemen's humanitarian crisis
12,000,000
people are food insecure
300,000
displaced from their homes
- 800 civilians killed since Saudi-led air campaign against Houthi rebel movement began on 26 March
- 1,500 civilians injured, with many lacking access to basic medical care
Enough food for more than 33,000 people was handed out in Hajjah, and the distribution of non-food items got under way in Amran and Hajjah, the OCHA added.
Two UN-chartered cargo ships carrying 420,000 litres of fuel have docked in Hudaydah. The fuel will be distributed to 50 organisations to support humanitarian operations, which are estimated to require five million litres of fuel every month.
Meanwhile, the deputy chief of staff of Iran's armed forces has warned coalition naval forces not to block an Iranian aid ship bound for Yemen under naval escort. Gen Masoud Jazayeri said such a move would "spark a fire" in the region.
The US, which is providing intelligence and logistical support to the coalition, has urged Iran to divert the ship to Djibouti, where the UN has set up an aid hub.
Saudi Arabia has accused Iran of providing military assistance to the Houthis, a charge both deny.
The UN says at least 828 civilians have been killed and 1,511 injured since 26 March, when the coalition launched air strikes targeting the Houthis and allied security personnel loyal to the ousted former president, Ali Abdullah Saleh.
