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 start of a handover of security responsibility from foreign forces to Afghans. The process is to begin in July and be complete by 2014. U.S. officials said it would be the focus of Gates's trip.

But civilian casualties have clouded the relationship and diverted attention from transition plans, with blunt exchanges between Karzai and U.S. leaders after a string of recent accidental killings.

Karzai said in a statement on Sunday 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.

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.

Petraeus again apologised 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.

Hundreds of Afghans chanting Death to America gathered in the capital on Sunday in protest. There have been at least four similar incidents, mainly in the east, in the past three weeks.

HAMPER EFFORTS

International concern over civilian casualties has grown, and the fallout from the recent incidents threatens 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 combat troops to leave by 2014. Karzai will announce on March 21 where and when the district-by-district, province-by-province transition will begin.

A senior U.S. defence official, speaking on condition of anonymity, said NATO and Afghan forces were in the process of identifying provinces, districts and municipalities that were ready to be put formally under Afghan control, even if foreign forces remain close at hand.

Gates is expected to visit parts of southern and eastern Afghanistan, where NATO commanders say they have weakened the Taliban and created bubbles of security they hope to link up.

In 2010, violence hit its worst levels since the Taliban were toppled in late 2001, and bloodshed has spread out of Taliban strongholds in the south and east into previously quiet areas in the north and west.

NATO forces have also been unable to halt the flow of militants across the porous eastern border with Pakistan.

Two key metrics for the transition will be the readiness of Afghan security forces to take over and an expected spring offensive by the Taliban.

Obama, who sent an extra 30,000 troops to Afghanistan after he overhauled his Afghan strategy in late 2009, has promised to start bringing U.S. soldiers home in July.

But the Pentagon has so far been quiet on how many soldiers, and what kind, will start to be pulled out.

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