The Pentagon says "tangible" progress has been made in the war in Afghanistan, with the Taliban routed from key southern strongholds.
But in its semiannual report to Congress, the Pentagon also notes that progress is fragile, with difficult fighting ahead as insurgents try to regain momentum and territory from international and Afghan troops.
The report was released Friday and covers the period between October of 2010 and the end of March. The Pentagon attributes an increase in violence during that time to a greater troop presence, more aggressive operations against Taliban safe havens, and a mild winter.
The Pentagon also notes challenges, including a shortage of military trainers to support Afghan forces as they prepare to take over security responsibility from foreign troops beginning this year.
U.S. President Barack Obama says U.S. forces will begin withdrawing from Afghanistan in July, and Afghan President Hamid Karzai has already announced seven regions where Afghan forces will take control from NATO troops.
The Pentagon also warns that political challenges and slow progress in improving governance could jeopardize gains made in security.
Friday's report came as NATO and U.S. military commanders warned that Taliban insurgents, aided by the al-Qaida-linked Haqqani network, were planning a series of violent attacks across Afghanistan over the next few days.
The commanders, who did not want to be named, said Friday the assessment was based on recent intelligence reports indicating the possibility of suicide bombings and other high profile attacks, likely concentrated in the east.
The officials say the increase in violence is aimed at casting doubts on recent gains made by the coalition.
Some information for this report was provided by AP, AFP and Reuters.