Somalia has said that an unnamed country helped plan Saturday's bomb explosion in Mogadishu that killed at least 90 people, and added it would investigate the assault with help from a foreign intelligence agency.

The bombing was the deadliest in more than two years in a country wracked by nearly three decades of terrorism and clan warfare.

"A foreign country planned the massacre of the Somalis in Mogadishu on 28 Dec 2019," the National Intelligence and Security Agency said in a tweet Monday.

NISA said it would use assistance from an unnamed foreign intelligence organization in its investigation.

Meanwhile, later Monday, the al-Qaida-allied al-Shabaab terrorist group claimed responsibility for the bombing in an audio message.

"The blast targeted a convoy of Turkish and Somali forces and they suffered great loss," Ali Mohamud Rage, al-Shabaab's spokesman, said in the message. Rage accused Turkey of "taking all resources of Somalia" and vowed to continue targeting their personnel in the country.

"We shall always fight ... the Turkish who work with the apostate government of Turkey. We are not against innocent Turkish Muslim citizens," he said.

The group carries out frequent bombings to try to undermine Somalia's central government, which is backed by the United Nations and African Union peacekeeping troops.

The most deadly attack blamed on al-Shabaab was in 2017 when a truck bomb exploded next to a fuel tanker in Mogadishu, killing nearly 600.

In recent years, Somalia has become an arena for military and diplomatic rivalry between Turkey and Qatar on one side, with Saudi Arabia and the United Arab Emirates on the other.