The minibus was travelling from Nahr-e-Saraj district to the provincial capital of Lashkar Gah when it hit a mine and all 19 people inside were killed on Friday, said Kamaludden, head of the National Directorate of Security in Helmand.
When security forces arrived at the scene they were attacked by Taliban insurgents, said Hekmatullah Akmal, a highway police commander.
He said it appeared at least 17 people had been killed, but the devastation caused by the blast made it difficult to determine how many people had been on the bus.
In Garmsir district, south of Lashkar Gah, a tractor also hit a roadside bomb, killing four civilians, officials said.
The first half of this year was the deadliest six months for civilians in Afghanistan since the near decade-old war began, according to the U.N. mission in Afghanistan.
Civilian deaths between January and June were up 15 percent compared to the first half of 2010, due to roadside and suicide bombings, increased ground fighting and more deaths from air strikes.
The minibus and tractor deaths in Helmand province come a day after at least 17 civilians were killed, including 12 children aged between four and 13, when suicide attackers targeted government buildings in neighbouring Uruzgan province.
Two policemen were also killed in that attack.
Helmand has been the site of some of the most vicious fighting of the war. Far more foreign troops have died there than in any other province and there are still several Helmand districts dominated by the Taliban.
Afghan security forces took over security control this month for Lashkar Gah, the most contentious of the first seven areas for which foreign troops handed over responsibility. The gradual transition of security control to Afghan forces is due to be completed by the end of 2014.