Afghan insurgents have claimed responsibility for the killing of four U.S. troops in southeastern Afghanistan.
The U.S. military reported in a press statement that the four deaths took place Saturday in Afghanistan's Zabul province.
The statement identified the dead as members of a Special Operations Task Force, but did not give further details.
The incident marks one of the largest one-day death tolls, since the end of the U.S.-led war that ousted Afghanistan's hard-line Taleban rulers.
A commander with the Taleban, remnants of which are leading an anti-government insurgency in the south and east of the country, claimed responsibility for Saturday's attack.
The commander said his fighters used rocket-propelled grenades to destroy two U.S. tanks north of the provincial capital of Qalat, and that the Taleban suffered no casualties in the engagement.
News of the engagement comes as Afghan officials reported fighting between government and Taleban forces in the southern province of Helmand.
The officials say fighting Saturday night left four government soldiers and one suspected Taleban dead, with a second Taleban fighter captured.
United Nations spokesman Manoel de Almeida e Silva told reporters Sunday that the Taleban and others still pose a serious threat to Afghan security.
"The threats to security continue," he said. "They continue because of the action of what we call the "spoilers," those who oppose this process, those who do not want to see a new Afghanistan."
The spokesman also cited fighting among rival local militias, whose commanders, although allied with the government, are accused of acting as independent warlords, ruling territory under their control as mini-states.
He said the situation must improve in order to register voters for the country's first post-war elections.
The elections, originally slated for next month, have been tentatively postponed for September, due to the security problems.
On Saturday, meanwhile, the United States said that the death last month of U.S. Special Forces soldier Pat Tillman, a former professional football player, was likely the result of friendly fire.
Mr. Tillman was killed while on patrol in the eastern Afghan province of Khost.