There was no immediate information on the number of victims, although witness accounts suggest that there were casualties.
According to a nurse, five bodies were brought to the hospital mortuary and 15 wounded people are being treated.
Police arrested nine suspects related to the probe Friday throughout Italy. Another nine were being sought.
3 people were wounded when a suicide bomber struck the gate of the compound in Lashkar Gah late Sunday: police
It was unclear whether the attack was linked to the insurgency led by the Afghan Taliban.
No one claimed the attack, which bore the hallmarks of Islamist insurgent group Boko Haram.
Police officials said on Saturday the attack in the town of Sheikh Zuweyid wounded two police officers.
A police officer at the scene of the latest bombing said four bodies and 48 injured people had been taken away.
Witnesses said the blast went off in the packed market neighbourhood of Kasuwar Arawa.
A Nigerian security source said the bomb went off at 12:15 p.m.
The bomber blew himself up near a line of Sunni pro-government fighters waiting for their salaries.
Kabul criminal investigation police chief says first attacker hit a bus with Afghan National Army officers, killing seven.
The attack took place near the town of Afgoye, some 30 kilometres northwest from the capital Mogadishu.
The bombing was followed by clashes with militants who tried to overtake the town.
Worst bombing since 2001; over 40 injured.