By Missy Ryan
KABUL (Reuters) - U.S. Defense Secretary Robert Gates arrived in Afghanistan on Monday at a time of increased strain between Kabul and its Western backers and with important security transition milestones looming.
Gates, whose visit was not announced in advance, will meet President Hamid Karzai, who complained angrily last week after nine Afghan children were mistakenly killed by helicopters from the NATO-led force.
Karzai will soon unveil a timetable for the beginning of the gradual handover of security responsibility from foreign forces to Afghans. The process is to begin in July and be complete by 2014.
But civilian casualties have clouded the relationship and taken attention away from transitions plans, with blunt exchanges between Karzai and U.S. leaders after a string of recent accidental killings.
Karzai on Sunday said in a statement that a rare and candid apology by General David Petraeus, commander of U.S. and NATO forces in Afghanistan, was "not enough."
The boys were gunned down while collecting firewood in a volatile eastern province last Tuesday. U.S. President Barack Obama has expressed his "deep regret" over the incident.
At a meeting with security advisers on the eve of Gates's trip, at which Petraeus was present, Karzai said civilian casualties caused by foreign troops were "no longer acceptable."
During the meeting, Petraeus again apologized for the killings, saying they were a "great mistake," according to a statement released by the presidential palace.
Karzai in turn said that the apology was not enough and that civilian casualties caused by NATO-led forces were the main cause of strained relations between the United States and Afghanistan, the statement said.
Earlier on Sunday, hundreds of Afghans chanting "Death to America" gathered in the capital to protest against such casualties. There have been at least four incidents involving civilian casualties, mainly in the east, in the past three weeks.
International concern over civilian casualties has grown, and the fallout from the recent incidents is even threatening to hamper peace and reconciliation efforts, with a gradual drawdown of the 150,000 foreign troops to begin in July.
U.S. and NATO leaders have agreed to Karzai's ambitious timetable for foreign forces to remove combat troops by 2014. Karzai will announce on March 21 where and when that district-by-district, province-by-province transition will begin.
Violence across Afghanistan is already at its worst since the Taliban were ousted in late 2001, despite the presence of a record number of foreign troops, and worse is expected with a spring surge in fighting.
(Writing by Paul Tait; Editing by Alex Richardson)