Defense Secretary Robert Gates flew into Kabul on Monday to assess the US-led war effort amid tensions with Afghan President Hamid Karzai over the deaths of nine children in a NATO air strike.
In an unannounced trip, Gates planned to visit troops in Afghanistan's east and south and confer with Karzai and top commander General David Petraeus, press secretary Geoff Morrell told reporters travelling with the Pentagon chief.
But his trip was likely to be dominated by Afghan anger over the deaths of nine children killed by coalition helicopters in the eastern province of Kunar last week.
A day before Gates' arrival, Karzai told Petraeus at a cabinet meeting that his public apology over the incident was "not enough."
"The civilian casualties are a main cause of worsening the relationship between Afghanistan and the US," a statement from the Afghan presidency quoted Karzai as saying.
Civilian casualties from coalition military operations against Taliban insurgents have long been a source of friction with Kabul, and have aggravated difficult relations between Washington and Karzai.
The incident came at a sensitive time as officers in the NATO-led force work with their Afghan counterparts on plans -- due to be unveiled later this month -- for a transition that will see Afghan forces take over security from foreign troops in some areas.
The handover will be accompanied by the start of a gradual US troop withdrawal in July, promised by President Barack Obama.
But Gates has yet to indicate how many of the 97,000 American forces in the country might be withdrawn.
"This is not a decision-making trip," Morrell said. "This will certainly inform him (Gates) on making those decisions in coming months."
It is Gates' 13th trip to Afghanistan, who was last in Kabul in December and who plans to step down later this year.
Morrell said that "we are going to go south, we are going to go east, and he will come away from this visit hopefully with a better sense of how far we've come in the past three months."
Gates and military commanders say coalition forces, backed up by a surge of 30,000 US reinforcements, have made headway against the Taliban over the last several months, pushing insurgents out of strongholds in the southern provinces of Kandahar and Helmand.
But the US-led campaign in the south will face a crucial test in the spring and summer, when the Taliban traditionally launch offensives.