Afghan President Hamid Karzai strongly condemned an air strike by NATO-led forces which killed 10 campaign workers for this month's election, a sour note as U.S. Defense Secretary Robert Gates arrived for talks.
Civilian casualties caused by foreign forces have been a major source of tension between President Hamid Karzai and Western nations.
Last month, a United Nations report said civilian casualties had risen by 31 percent in the first six months of 2010 compared with the same period last year.
Thursday's attack happened in the Rostaq district of Takhar, a relatively peaceful province in the north near Tajikistan, said a spokesman for the provincial governor.
Spokesman Faiz Mohammad Tawhidi said the candidate, Abdul Wahid, and some of his supporters were wounded in the air strike, which Tawhidi said included two helicopters and two fixed-wing aircraft.
"Six of his campaigners have been killed in the aerial attack," Tawhidi said.
He said the number of casualties could rise and that he had been told of the strikes by security officials.
Visit of Gates
Gates flew into the Afghan capital from Iraq, where he attended ceremonies to mark the end of U.S. combat operations.
Gates will also meet General David Petraeus, commander of U.S. and NATO forces in Afghanistan and U.S. ambassador Karl Eikenberry, as well as visiting U.S. troops.
Civilians have borne the brunt of the atacks.
Wikileaks unveiled secret U.S. military documents that showed thousands of uncovered civilian killings.
A spokesman for the NATO-led International Security Assistance Force (ISAF) said investigations were under way.
There are no foreign troops stationed in Takhar, according to an ISAF troop distribution map, (www.isaf.nato.int), but German units are based in Kunduz to the west and Badakhshan to the east.
Last September, a U.S. air strike called in by German troops killed scores of civilians in Kunduz. The strike led to the resignation of the German defence minister.