A militant ambush of an opium poppy eradication force sparked clashes that killed 25 Taliban fighters and a policeman, a provincial authority said Thursday. Four other militants died when a bomb went off.

Insurgents ambushed the drug eradication force Wednesday in Marja district of Helmand province, killing one police officer and wounding two, said Gen. Mohammad Hussein Andiwal, the provincial police chief.

Police launched an attack afterward, killing 25 Taliban fighters, including a senior regional militant commander, the Interior Ministry said in a statement.

Helmand, a front line between militants and foreign forces, is the world's largest opium-producing region. Officials estimate that up to 40 percent of proceeds from Afghanistan's drug trade — an amount worth tens of millions of dollars (euros) — is used to fund the insurgency.

Separately, four militants died and another was wounded Thursday when the roadside bomb they were planting on a main road in Helmand province exploded prematurely, Andiwal said. Militants regularly target Afghan and foreign troops with roadside bombs, though many civilians are killed by the blasts.

Last year was the deadliest in Afghanistan since the 2001 U.S.-led invasion. More than 6,500 people — mostly militants — were killed in insurgency-related violence, according to an Associated Press count.

The top U.S. intelligence official told a Senate committee in Washington on Wednesday that Afghanistan's central government, headed by President Hamid Karzai, controls just 30 percent of the country.

The resurgent Taliban controls about 10 percent of the country, while local tribes control the rest, according to National Intelligence Director Michael McConnell.