They say at least 120 others were wounded in Sunday's bombardment, which came amid attempts by government forces to tighten their grip on the city.
Aden has seen months of heavy fighting between rebels and loyalists.
Pro-government forces, backed by air strikes from a Saudi-led coalition, last week recaptured most of the port city, including the airport.
On Sunday, fierce clashes raged in Aden's northern district of Dar Saad.
A Houthi rebel leader denied bombarding the district, but Yemeni medics said the shelling came from the rebel-held areas, the Associated Press news agency reports.
Overnight pro-government militias failed to capture the last remaining rebel stronghold in the city - the Tawahi peninsula in the south.
The government fled Aden in March, as the Houthis - Shia Muslims from the north who control the capital Sanaa - pushed towards the provincial capital.
Following the recent advances by loyalist forces, the exiled government of President Abdrabbuh Mansour Hadi declared the area "liberated" on Thursday.
Correspondents say the losses in the Aden region represent the biggest setback for the Houthis since Sunni power Saudi Arabia launched a coalition air campaign against them in March.
The Saudis say the campaign is aimed at forcing the rebels - who they say are armed by regional Shia rival Iran - to retreat and to restore the exiled government.
Iran denies backing the Houthis militarily.
The UN says more than 3,200 people, half of them civilians, have been killed in the conflict.
Another one million civilians have been displaced by the conflict and more than 80% of Yemen's 25 million people now need some form of humanitarian aid.
Why is there fighting in Yemen?
- Northern Shia Muslim rebels known as Houthis, backed by forces loyal to Yemen's ex-president, took over parts of Yemen, including Sanaa, and forced the government into exile in March
- The rebels accused the government of corruption and of planning to marginalise their heartland within a proposed federal system
- Forces loyal to the government, and southern militia, are fighting back, aided by air strikes led by neighbouring Saudi Arabia