The death toll from Saturday's truck bomb blast in Somalia's capital, Mogadishu, has surged to at least 276 people, according to the country's information minister.

Abdirahman Osman on Sunday said on Twitter that around 300 others were also wounded in the powerful explosion at a busy road junction, which flattened nearby homes and businesses and turned vehicles into burned wrecks.

He called the attack "barbaric" and said a number of countries, including Turkey, Ethiopia and Kenya, had already offered to provide medical assistance.

President Mohamed Abdullahi Mohamed Farmaajo on Sunday declared three days of national mourning following the attack.

"We will observe three days of mourning for innocent victims, flags will be flown at half-mast. Time to unite and pray together. Terror won't win," Mohamed said in a statement posted on the presidency's official Twitter account early on Sunday.

The president also urged residents to help those affected by the attack.

"I call on our citizens to come out, extend help, donate blood and comfort the bereaved. Let's get through this together," Mohamed said.

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The blast, described by Mogadishu residents as the most powerful they had witnessed in years, also left dozens wounded.

No group has claimed responsibility for the attack.

Emergency services were overstretched and worked late into the night as they tried to rescue people who were trapped in the destroyed buildings.

The city's mayor called on residents to donate blood as hospitals were running out of blood.

"I call on the Somali people to visit the city's hospitals and donate blood. Please, come to the rescue of your brothers," Thabit Abdi Mohammed, told reporters after donating blood at a local hospital.

The attack came 48 hours after both the defence minister and army chief of the country resigned from their posts without explanation.