At least 15 people were killed by a suicide bomber from the Al-Qaeda linked Shebab, officials said Thursday, as details emerged of the heavy blast in Somalia's war-torn capital Mogadishu.

"The horrible suicide blast, carried out by desperate and violent criminals, killed at least 15 innocent civilians, while 20 others are injured," said Mogadishu mayor Mohamed Ahmed Nur, updating an initial toll of 11.

Nur said two lawmakers were also slightly injured in the blast Wednesday afternoon at a small cafe where people had gathered to drink tea, in the heart of the government quarter and close to the presidential palace.

"They are cowards, attacking soft targets -- innocent people drinking tea," Nur said of the attack, the latest in a string of blasts including roadside bombs and grenade explosions that have rocked the Somali capital.

Extremist Shebab fighters said their attack had targeted government officials at the nearby Mona hotel, a site infamous for an August 2010 attack when Shebab suicide gunmen killed 32 people including six lawmakers.

"The mujahedeen fighters carried out a bomb attack against Mona hotel, where the apostate lawmakers and security officials stay," said Shebab spokesman Sheikh Abdulaziz Abu Musab. "We will continue targeting them everywhere."

Mogadishu has seen an increase in such attacks since the hardline Shebab abandoned fixed positions there in August and switched to guerrilla tactics against the Western-backed government and African Union troops.

The explosion was the deadliest in the war-torn city since October, when a truck packed with explosives killed at least 82 people.

The attack took place hours after European Union Special Representative for the Horn of Africa Alexander Rondos visited the war-wracked city, where he met officials from the embattled government at the presidential palace.

Jerry Rawlings, the former Ghanaian leader and African Union special envoy to Somalia, had also visited the palace on Wednesday.

The attack comes two weeks ahead of a conference in London that is supposed to address the various problems besetting Somalia, and the British ambassador to Somalia Matt Baugh condemned the "barbaric" attack in a statement from Mogadishu.

"This event will only strengthen our resolve to work harder in helping the Somali people to build a better future that is free from violence and extremism," Baugh said.

Somalia has been without an effective central government since 1991 and the government in Mogadishu is propped up by a 10,000-strong African Union force from Uganda, Burundi and Djibouti.

Hardline Shebab insurgents control large parts of central and southern Somalia, but are facing increasing pressure from government forces and regional armies.

Armies from neighbouring countries are converging on the Shebab -- Kenyan forces in the south, Ethiopia's army in the south and west, and the AU troops in Mogadishu.

The United Nations says Somalia is suffering one of the world's worst humanitarian crisis.