Saudi-led coalition aircraft have carried out fresh air strikes on Yemen's capital, Sanaa, hours before a five-day ceasefire is set to begin.

An arms depot used by the Houthi rebel movement in the city's north-east was targeted for a second day, sending a column of smoke into the sky.

The coalition also bombed rebel positions in the southern city of Aden.

The proposed truce to allow deliveries of desperately needed humanitarian aid is due to start at 23:00 (20:00 GMT).

However, Saudi Arabia has said its offer of a pause in air strikes is conditional on the Houthis reciprocating and not exploiting the ceasefire for military advantage.

The Houthis have agreed to the truce, but said they will "respond" to any violations.

Civilians trapped

On Tuesday, coalition aircraft bombed the arms depot at a military base on Mount Noqum in the east of Sanaa for the second consecutive day, witnesses said.

Explosions caused by two strikes on the depot on Monday sent debris crashing down the mountainside onto a residential area, killing at least five people, according to AFP news agency.

Air strikes and fighting in Yemen have killed at least 182 civilians in the past week. Photo: Reuters

The coalition also bombed Houthi positions in Aden, and local militiamen allied to Yemen's exiled President Abdrabbuh Mansour Hadi continued to fight the rebels in the port city and elsewhere in the country's south, Reuters news agency said.

There were no immediate reports of casualties from Tuesday's strikes.

But 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 Mr Hadi.

The six days from 4 to 10 May have been the deadliest, with at least 182 civilians reported killed, almost half of them women and children. A significant proportion of the casualties were caused by air strikes, especially in the Houthis' northern heartland of Saada province.

Militiamen loyal to the exiled president have been unable to halt the Houthis' advance. Photo: Reuters

Analysts say the coalition appears to be trying to inflict as much damage as possible on the Houthis and allied security personnel loyal to the ousted former president, Ali Abdullah Saleh, before the humanitarian ceasefire is scheduled to begin on Tuesday evening.

The new UN envoy for Yemen, Ismail Ould Cheikh Ahmed, has arrived in Sanaa, where he hopes to meet various parties, including the Houthis.

On Monday, the UN humanitarian chief Valerie Amos called on all sides in the conflict to "stop the fighting and bombing and give the people of Yemen respite".

"Given the deteriorating humanitarian situation on the ground in Yemen with hundreds of thousands of vulnerable civilians trapped in the middle of fighting and unable to access lifesaving aid it is essential that this pause materialise," a statement said.

Baroness Amos said two World Food Programme cargo ships arrived in the Red Sea port of Hudaydah over the weekend with fuel, food, water and nutritional supplies. Other supplies were ready to be brought in and planes were standing by to help evacuate the wounded, she added.

In a separate development on Tuesday, a jihadist website reported that four members of al-Qaeda in the Arabian Peninsula (AQAP) had been killed in a suspected US drone strike on Monday in the eastern Yemeni port of Mukalla.