The U.S. military said Monday that increased militant violence in Afghanistan was proving "very hard to combat" as separate attacks killed two police officers and a truck driver delivering food to coalition forces in a former Taliban stronghold in the south.

Gunmen also killed five medical workers before burning down their clinic late Sunday in a rare attack in the normally calm northwest.

The violence follows threats by Taliban militants to intensify attacks against U.S.-led coalition forces and Afghan troops during the spring and summer months.

In Kabul, U.S. military spokesman Col. James Yonts said Taliban forces have increased their attacks and changed tactics to spread a campaign of fear across the country rather than try to defeat the security forces militarily.

Some of the new tactics include roadside and suicide bombings, which Yonts said were proving "very hard to combat."

"They are doing it because it is successful. They have shifted their tactics to something that is successful," he told a news conference.

Much of the violence has taken place in the southern and eastern regions where the Taliban are strongest, but the killing of the medical workers in Badghis, 230 miles northwest of the capital, was unusual because it occurred in a province that has been largely peaceful.

Gunmen stormed the workers' clinic and killed everyone inside, including a doctor and several nurses, before burning the building down, provincial Gov. Hanayatullah Hanayat said. No patients were in the clinic, and security forces are investigating the slayings.

"This clinic was essential for this area," Hanayat said. "It was the only health care there."

Separately, a bomb blast killed two policemen and wounded two others Monday during an opium eradication patrol in the southern Helmand province, the country's main poppy growing region, provincial police chief Gen. Abdul Rahman Saber said.

Afghanistan supplies some 90 percent of the world's opium and heroin, and some of the profits from the illicit business are believed to go to the Taliban. Security forces have been deployed in a major campaign to destroy poppy fields in Helmand amid concerns that cultivation is increasing.

Elsewhere in Helmand, Taliban militants stopped two Afghan trucks transporting food for coalition forces, killing one driver, while the other fled in his vehicle, said Girishk district police chief Maj. Amanullah, who uses only one name.

Separately, about 40 Taliban militants attacked a government building in Waygal district in the remote eastern province of Nuristan late Sunday, and three of the group's fighters were wounded, said Gen. Abdul Baqi, the provincial chief of police.

Some of the Taliban rebels wore Afghan army uniforms in the attack, which ended with the fighters escaping into the night. No Afghan forces were wounded.

The escalating violence, more than four years after the hard-line Taliban regime was driven from power and after two democratic elections, is a worry for the United States, which has more than 18,000 troops in Afghanistan.

It also is a growing concern to other nations contributing troops under the mandate of NATO, which is doubling its current force of 10,000 troops to about 21,000 by November, as it gradually assumes command of all the international forces in the country. Some 6,000 mainly British, Canadian and Dutch troops have started moving into the rebellious southern provinces.