U.S. soldiers and Afghan police backed by war planes and helicopters clashed with insurgents in the southeastern mountains near Pakistan, and about 20 suspected militants and one policeman were killed, the U.S. military said Wednesday. Six Americans were wounded. Tuesday's clash, one of the deadliest in recent months, occurred in Dehchopan district of Zabul province, the U.S. military said in a statement. Zabul lies in a swath of Afghan territory along the border with Pakistan where Taliban-led militants opposed to the government of U.S.-backed President Hamid Karzai have revived their 3-year-old insurgency after a winter lull. Clashes and bombings in recent weeks have killed dozens of rebels and Afghan forces, several civilians, a U.S. soldier and a Romanian soldier. The military said the American soldiers wounded Tuesday were in stable condition, and two of them had returned to duty. The other four would be flown to a U.S. military hospital in Germany for treatment.The fighting began when gunmen fired on a group of U.S. soldiers and Afghan police investigating a reported beating of an Afghan man, the military said. U.S. troops and the policemen "cordoned off the insurgent forces by use of small-arms fire and support from coalition fixed-wing and rotary-wing aircraft," it said. "Initial reports indicate approximately 20 insurgents were killed and one wounded," the statement said. The statement said five Afghan police officers also were wounded, but it cautioned that the casualty figures were still preliminary. Six insurgents were detained. The U.S. military gave no details on the identities or affiliations of the suspected insurgents killed Tuesday but said a local leader also was detained after villagers "reported him as a Taliban member." The military said it was trying to organize assistance to the village, which was not identified.