US Defence Secretary Robert Gates expressed concern Thursday about the rise in attacks in Afghanistan and Pakistan's inability to stop militants from crossing the border, the dpa reported.
"The ability of the Taliban and other insurgents to cross that border and not being under any pressure from the Pakistani side of the border is clearly a concern," Gates told reporters.
The Pakistani government has been unable to control the northwest frontier tribal area used for refuge by the Taliban, frustrating US commanders in Afghanistan.
"It is a matter of concern, of real concern, and I think that one of the reasons that we're seeing the increase ... is more people coming across the border from the frontier area," Gates said.
The issue arose last week after the Taliban launched a massive assault in eastern Afghanistan that had to be fought off by US, NATO and Afghan forces.
The United States has been sceptical of plans by the Pakistani government to negotiate a ceasefire with the militants over concerns it will allow them to continue operating safely in Pakistan.
Gates, however, welcomed Pakistani Prime Minister Yousaf Raza Gilani's decision Wednesday approving the use of force to clear the tribal area of militants to pressure them at the negotiating table.