August 2011 has become the deadliest month for US troops in the nearly 10-year war in Afghanistan, according to an unofficial tally.
The Associated Press counted 66 US deaths in the month, including 30 killed when their helicopter was shot down by Taliban insurgents.
But across the year, casualties are down slightly from 2010.
President Barack Obama plans to withdraw 33,000 troops from Afghanistan by next summer.
"As our mission transitions from combat to support, Afghans will take responsibility for their own security, and the longest war in American history will come to a responsible end," President Obama told a veterans event on Tuesday.
Last month, the newly installed Nato commander in Afghanistan, Gen John Allen, warned of "tough days ahead" as insurgents step up attacks on coalition forces.
The 30 US troops - mostly Navy Seal special forces - died when their Chinook helicopter was shot down by a Taliban insurgents on 6 August, marking the single largest loss of US life in the war.
In the first eight months of this year, 306 US troops died in Afghanistan, according to a tally by iCasualties.org.
That is down about 5% from the first eight months of 2010, when 321 US troops were killed.
This year 10,000 troops are to be withdrawn from Afghanistan, with another 23,000 pulled out in 2012. The move will leave about 68,000 US troops there.
Foreign troops are expected to end combat operations in Afghanistan by 2014.