Somali insurgents killed at least five people Friday in multiple attacks in the capital Mogadishu, with a suicide bomber ramming an African Union convoy while later a grenade was thrown into a hotel.

Police official Ahmed Muktar said that three people were killed when a car laden with explosives smashed into the armoured AU convoy, the latest in a string of explosions in the dangerous capital.

Al-Qaeda linked Shebab fighters claimed members of their suicide brigade carried out the attack, calling it "a martyrdom operation targeting a convoy of crusaders".

The hardline fighters boasted it would be "one of many that will make the kuffar (unbeliever) relinquish all hope in Mogadishu," they said in a statement.

In previous years, the Muslim holy month of Ramadan -- which began this week -- has seen a surge in Shebab attacks, with gunmen urged on by extremist preachers.

"I saw a car speeding towards the convoy, then it was a huge explosion, buildings all around were rocked by the blast," said Hussein Gure, a witness who was driving nearby when the car exploded.

Witnesses said that a mass of roadside shacks near the blast site had been reduced to rubble.

The AU mission confirmed a car packed with explosives "attempted to hit" one of their armoured troop transport vehicles along one of Mogadishu's main central highways, but that none of its troops were killed in the blast.

The Shebab have carried out a series of bombings and killings aimed at overthrowing the internationally-backed government.

Later on Friday, unknown insurgents hurled a grenade into the Barwaqo hotel in central Mogadishu, killing two people and wounding at least 10.

"There was a loud explosion, people were wounded, two people were killed," said Ahmed Yasin Ibrahim, who was near the hotel during the attack.

Prime Minister Abdi Farah Shirdon called the attacks "acts of terrorism" but vowed that they "will not derail the progress made in Mogadishu and across Somalia."

In the past year, the Shebab have lost a string of towns to the 17,700-strong AU troops, which fights alongside government soldiers.

"The enemies of peace show their true colours to the world," Shirdon added. "Muslims killing Muslims in the holy month of Ramadan."

Shebab spokesman Abdulaziz Abu Musab also claimed responsibility for a bomb beneath a fuel tanker belonging to the AU force in the town of Afgoye, northwest of Mogadishu.

That attack could not be immediately confirmed.

Despite recent infighting inside the Shebab -- including the recent killing of top leaders in a bloody purge -- analysts warn the extremist group is far from defeated.

Last month Shebab suicide commandos carried out a brazen daylight attack on a fortified United Nations compound.

Key Shebab strongholds remaining include rural southern and central Somalia, while another faction has dug into remote and rugged mountains in the northern, semi-autonomous Puntland region.