The suicide bomber detonated his explosives-rigged truck near a complex that houses the governor, the head of the provincial council and the deputy provincial police chief in Lashkar Gah, the capital of Helmand province, a senior local official said.
All three survived, but "seven civilian were killed and 41 were wounded in the attack", Deputy Provincial Governor Mohammad Jan Rasolyar told AFP.
"The attack was targeted at a gathering of senior government officials, civil society members and journalists in a hall for a seminar on human trafficking and kidnapping," he added.
A senior police officer, who declined to be named, confirmed the attack and casualties, adding that buildings in the complex were also damaged. The interior ministry also confirmed the attack in a statement.
Also Read
Afghanistan's military began a offensive against Taliban fighters in Helmand province last month, in what was seen as a key test of their ability to curtail the insurgency following the end of US-led NATO combat mission in late December.
On Monday, it claimed the killing of militant commander Hafiz Waheed, who led a faction Kabul claims is linked to the Islamic State group -- though the organisation has never acknowledged having affiliates in Afghanistan.
The number of civilians killed and wounded in Afghanistan jumped 22 percent in 2014, the UN said last month, as NATO troops withdrew from combat.
The United States was due to reduce its 10,000 troops to 5,500 by December, but that number is expected to be revisited soon, according to US officials.
The Taliban, who frequently target Afghan government officials and security forces, claimed responsibility for the attack through their official Twitter account.