Fighting gripped in Yemen's second city Aden on Sunday as the UN envoy was expected in the rebel-held capital Sanaa to press his efforts to broker a ceasefire.

Saudi-led warplanes bombed rebel positions, killing eight people, while rebel rocket fire killed six, including a child, officials said.

Aden was the last refuge of President Abedrabbo Mansour Hadi before he fled to Saudi Arabia in March and has been a key battleground ever since.

In neighbouring Lahj province, Hadi loyalists attacked a rebel gathering, killing 11, military sources said.

They also attacked the rebel-held Al-Anad air base, Yemen's largest. Eight rebels and two Hadi loyalists were killed, the sources said.

UN envoy Ismail Ould Cheikh Ahmed was due in Sanaa for talks with the rebels and their allies on his ceasefire plans.

After talks in Riyadh with the exiled government, he expressed optimism on Wednesday that a humanitarian truce could be agreed.

The same day, the United Nations declared Yemen a level three emergency, the highest on its scale.

More than 21.1 million people -- over 80 percent of Yemen's population -- are in need of aid, with 13 million facing food shortages.

More than 2,800 people have been killed in the impoverished Arabian Peninsula country since March, according to UN figures.