Lahore, July 16 : Pakistan's security controls on its border with Afghanistan are a "real concern" for the US, following an attack two days ago that killed nine US soldiers at a camp near the frontier.
"There is a deep concern about cross-border infiltration into Afghanistan and then back (across) the border," State Department spokesman Sean McCormack told reporters.
Pakistan understands "the importance of engaging in the counter-terrorism fight," he added.
The soldiers were killed in Kunar province in north-eastern Afghanistan, the NATO-led International Security Assistance Force said in a statement.
A group of about 200 gunmen raided a US post in the deadliest attack on US forces in Afghanistan in three years.
The Pakistan government said that it is combating extremism through the selective use of force and a strategy of economic and political development in the tribal regions.
Twenty-eight coalition soldiers were killed in June in the deadliest month for the US-led forces in Afghanistan since the conflict began in 2001, the US Defence Department said earlier this month.
Terrorist incidents in eastern Afghanistan were 50 percent higher in April than the same month last year, according to NATO.
NATO and US commanders are trying to address border security with Pakistan's Army, Pentagon spokesman Bryan Whitman said in Washington on Monday.
Afghan President Hamid Karzai on Monday blamed Pakistan's intelligence agencies for being behind recent attacks by militants.
Pakistan has repeatedly denied any involvement, saying it wants to work with Afghanistan to defeat terrorism, the Daily Times reported.