Mohammad Hussein Andiwal, the police chief of Helmand province, said the bombing in Gereshk district also wounded 17 people, including five children.
Andiwal said there were no security forces in the area and accused the Taliban of targeting civilians to "create fear in the people."
The Taliban frequently target Afghan and international security forces, but this year have increased their attacks against civilians as well.
A statement Tuesday attributed to Taliban senior commander Mullah Bradar threatened more attacks this spring using new techniques, and also warned Afghans working with the government to quit their jobs or risk being targeted.
Bradar said the Taliban is seeking the collapse of President Hamid Karzai's government. He said they would continue their attacks until the government is ousted and U.S. and NATO forces withdraw.
U.S. and NATO military officials dismiss the idea of a Taliban spring offensive and say the only offensive that will take place this year in Afghanistan is one by Western and Afghan troops.
"It's the same old story, it's the same old nonsense," Mark Laity, the NATO spokesman in Kabul, said Wednesday. "What are they saying they will do? More destruction, more unhappiness, more misery. What is there that will present any hope for the Afghan people?"
Violence has risen during the warmer months of spring and summer in the past several years, usually through a spike in roadside and suicide bombs. The Taliban does not have the number of fighters or the military equipment needed to mount a conventional offensive against the U.S., NATO or Afghan troops.
Last year was Afghanistan's most violent since the 2001 U.S.-led invasion. More than 8,000 people were killed, including some 1,500 civilians, according to the U.N. Most of those deaths were of militants killed in U.S. and NATO strikes.
Gen. Mohammad Zahir Azimi, spokesman for Afghanistan's Defense Ministry, said the Taliban announcement was nothing but propaganda.
"In the past they've used all their power against the Afghan National Army, but they failed," Azimi said. "Thousands of Taliban were killed last year. The ANA has increased its numbers. Important Taliban leaders have been killed."
The Afghan army, which is being trained and equipped by U.S. and other NATO experts, now has 63,000 troops, Azimi said. The international community has agreed to expand the army to 80,000 troops, though Azimi has called for a force of 200,000.
Azimi also said the Taliban is suffering from infighting in its ranks, including disagreements between Taliban leader Mullah Omar and powerful Pakistani Taliban commander Baitullah Mehsud.
The director of U.S. national intelligence said in February that the Taliban control about 10 percent of Afghanistan, and a U.N. report this month said 10 percent of the country's districts are inaccessible to aid workers. Afghanistan's top intelligence chief has said only eight of Afghanistan's 364 districts are not under government control.