At least 22 people were killed Thursday in Yemen as fighting raged in the southern port of Aden and Saudi-led warplanes bombed Shiite rebels in the capital Sanaa, officials said.

The violence came a day after the UN declared its highest level of humanitarian emergency in the country, where it says some 3,000 people, half of them civilians, have been killed since March.

But there was a ray of hope, as the Red Cross said it had begun distributing food in the beleaguered city of Aden, where it said around half the people are displaced from their homes.

The clashes in Aden erupted at dawn in northern part of the port city, killing seven rebels and five pro-government fighters, a military official said.

Two civilians were also killed in rebel shelling of a western district that also damaged several homes, residents said.

The bloodshed comes a day after rebel rocket fire hit a residential district of Aden, killing 31 civilians and wounding more than 100.

Meanwhile, a port near the oil refinery came under rebel artillery fire for a fifth consecutive day, as a fire continued in the area, said Aden Refinery Co. spokesman Naser al-Shayef.

In Sanaa, warplanes pounded several positions of the Huthi rebels around midday, residents reported.

Medical officials said eight rebels were killed and at least 10 wounded in the strikes, which they described as the most violent since the Muslim fasting month of Ramadan began two weeks ago.

The coalition has been bombing the Iran-backed Huthis and their allies since March 26 in support of President Abedrabbo Mansour Hadi, who fled to Saudi Arabia.

Amnesty International warned of "the high price civilians continue to pay amid the... air strikes all over the country" and accused the coalition of failing to "abide by the requirements of international humanitarian law".

That requires belligerents to "take all possible steps to prevent or minimise civilian casualties," said Donatella Rovera, senior crisis response adviser at Amnesty.

But there is "no indication" the coalition has done "anything to prevent and redress such violations," said Rovera, who is currently in Yemen.

- 'Humanitarian catastrophe' -

On Wednesday, the United Nations declared Yemen a level-three emergency, the highest on its scale, as aid chief Stephen O'Brien held talks to discuss the crisis.

UN chief Ban Ki-moon also called anew for an "immediate end to the fighting in Yemen to help stem the unfolding humanitarian catastrophe in the country," a spokesman said.

Wednesday's statement said that, in addition to some 3,000 Yemenis killed since March, the war has also left 14,000 wounded and displaced more than a million people.

It said "21 million (over 80 percent of Yemen's population) need immediate help" and that "close to 13 million people are unable to meet their food needs," while "15 million people have no health care".

"Outbreaks of dengue and malaria are raging unchecked," it added.

The International Committee of the Red Cross said in Aden it had started distributing 160 tonnes of food across the front lines to cover the emergency needs of 17,500 people.

"Aden residents have been living under extreme duress for three months," says Robert Zimmerman, who heads a Red Cross operation there.

"In today's environment, providing something as basic as food remains a significant challenge, but we will continue to coordinate with all armed groups present in Aden to ensure that emergency aid is delivered impartially to those who need it," he said.

UNESCO, meanwhile, placed Sanaa and the southeastern city of Shibam on its list of endangered World Heritage sites due to the violence.

Sanaa, known for its many Islamic sites and ancient multi-storeyed mud brick buildings, has sustained damage in the conflict, said UNESCO.

Shibam, nicknamed the "Manhattan of the Desert" for its high-rise mud brick buildings, is also "under potential threat from the armed conflict," it added.

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