A double homicide bomb attack killed and wounded at least 20 people in southern Afghanistan on Sunday, an official said. The majority of casualties were police and army units responding to the initial attack.

The first homicide bomber detonated his explosives near a taxi stand and police checkpoint in the town of Gereshk in Helmand province, the world's largest poppy-growing region and the country's most violent province.

Helmand deputy provincial police chief Kamaluddin Khan said when police and army units responded to the scene, a second homicide bomber on foot detonated his explosives, causing a majority of the casualties.

A definitive casualty toll was not immediately available.

Khan said more than 20 people had been killed or wounded in the attacks, mostly police and soldiers.

Taliban militants have increased their attacks the last three years as the country's insurgency has turned increasingly bloody.

President Obama is sending 21,000 additional U.S. troops to the country to bolster the record 38,000 American forces already in the country.

In other violence Sunday, a roadside bomb in eastern Nangarhar province killed eight construction workers traveling on a rural road on their way to build a checkpoint for the country's border police, said Ahmad Zia Abdulzai, the spokesman for Nangarhar's governor.

Three Afghan civilians — a truck driver and two assistants — died in a roadside bomb blast in Zabul province while transporting goods to an American base, said Ghulam Jalani Khan, the deputy provincial police chief. Two police died in a separate blast on Saturday, he said.