The death toll from the most powerful bomb blast witnessed in Somalia's capital rose to 189 with more than 200 injured, making it the deadliest single attack ever in the Horn of Africa nation, police and hospital sources said Sunday. 

Doctors struggled to assist horrifically wounded victims, many burnt beyond recognition. Officials feared the toll would continue to climb from Saturday's truck bomb that targeted a busy street near key ministries. Sources for the death toll spoke on condition of anonymity as they were not authorized to speak to reporters.

Ambulance sirens still echoed across the city as bewildered families wandered in the rubble of buildings, looking for missing relatives. "In our 10 year experience as the first responder in (hash)Mogadishu, we haven't seen anything like this,'' the Aamin Ambulance service tweeted.

Grief overwhelmed many.

"There's nothing I can say. We have lost everything,'' wept Zainab Sharif, a mother of four who lost her husband. She sat outside a hospital where he was pronounced dead after hours of efforts by doctors to save him from an arterial injury.

President Mohamed Abdullahi Mohamed declared three days of mourning and joined thousands of people who responded to a desperate plea by hospitals to donate blood for the wounded victims. "I am appealing all Somali people to come forward and donate,'' he said.

"The hospital is overwhelmed by both dead and wounded. We also received people whose limbs were cut away by the bomb. This is really horrendous, unlike any other time in the past,'' said Dr. Mohamed Yusuf, the director of Medina hospital. 

Overnight, rescue workers with flashlights searched for survivors trapped under the rubble of the largely destroyed Safari Hotel, which is close to Somalia's foreign ministry. The explosion blew off metal gates and blast walls erected outside the hotel.

Somalia's government has blamed the al-Qaida-linked al-Shabab extremist group for the attack it called a ``national disaster.'' However, al-Shabab, which often targets high-profile areas of the capital with bombings, had yet to comment.

"They don't care about the lives of Somali people, mothers, fathers and children,'' Prime Minister Hassan Ali Khaire said. "They have targeted the most populated area in Mogadishu, killing only civilians.'' 

Somalia's information minister, Abdirahman Omar, said the blast was the largest the city had ever seen. "It's a sad day. This how merciless and brutal they are, and we have to unite against them,'' he said, speaking to the state-run radio station.

The United States joined the condemnation, saying "such cowardly attacks reinvigorate the commitment of the United States to assist our Somali and African Union partners to combat the scourge of terrorism.''

The U.S. military has stepped up drone strikes and other efforts this year against al-Shabab, which is also fighting the Somali military and over 20,000 African Union forces in the country.

Saturday's blast occurred two days after the head of the U.S. Africa Command was in Mogadishu to meet with Somalia's president, and two days after the country's defense minister and army chief resigned for undisclosed reasons. (AP)