Militants ambushed an opium poppy eradication force in southern Afghanistan, sparking clashes that left 25 Taliban fighters and a policeman dead, police 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 attacked the militants 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 — 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 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 violence linked to the insurgency, according to an Associated Press count.

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

The resurgent Taliban controls 10 percent to 11 percent of the country, while local tribes control the rest, National Intelligence Director Michael McConnell said.

Afghanistan's Defense Ministry rejected McConnell's assessment, insisting the government controls the vast majority of the country.

"This is far from the facts and we completely deny it," the Defense Ministry said regarding McConnell's testimony.

"All Afghan people know that in the 34 provinces of Afghanistan and in more than 360 districts ... the government has control," the statement said.

The Defense Ministry has previously said that several districts in Helmand are not controlled by the government. Afghanistan has about 365 districts.

In international diplomatic circles, Karzai is sometimes referred to as the "Mayor of Kabul," a reference to his control of the capital but weak authority in remote areas of the country.

Afghanistan's hundreds of tribes are an important element of the country's social fabric. Tribes tend to provide their own security through militias and administer justice and solve problems using traditional methods, such as large gatherings known as shuras.

"For a long time we know that tribal leaders were effective in ensuring security in their areas, and because of that we will give them opportunities and encourage them to provide security in their areas," said Asif Nang, a spokesman for the minister of state for parliamentary affairs. "But this does not mean that the government is not present."

Many of the tribes tend to be allied with the government but remain staunchly independent in running their affairs. Securing tribal loyalties is an important element of enabling the government to conduct work in certain areas of the country.

Recognizing that fact, the U.S. military has organized a number of gatherings with tribal elders. The approach, coupled with millions in aid and reconstruction projects, is part of the ongoing international effort to prop up the central government and create a police and army respected nationwide.