Adm Mullen (r) has been meeting Afghan leaders
More foreign fighters, including al-Qaeda militants, are operating in Pakistan's tribal region than before, America's top soldier has said.
Adm Mike Mullen also said Pakistan was not doing enough to stop militants from crossing over into Afghanistan.
Attacks on the US forces in eastern Afghanistan, on Pakistan's border, have grown rapidly in the last few months.
Adm Mullen, the chairman of the US joint chiefs of staff, visited Pakistan twice this year for anti-terror talks.
"There are clearly more foreign fighters in the Fata (Pakistan's Federally Administered Tribal Areas) than have been there in the past," Adm Mullen was quoted by news agency Associated Press as saying.
"There's a clear problem on the border," he added.
The chief of the US military made the comments on Thursday in Kabul. He is on a six-day visit to the region.
Adm Mullen said the new government in Pakistan was working its way through figuring out how to deal with the extremist challenge.
"There's clearly not enough pressure being brought to bear, particularly on the Pakistan side of the border. There's more freedom there," AP quoted him as saying.
Attacks on US troops fighting in eastern Afghanistan have grown in the past few months.
Many senior US officials have in the past been critical of the Pakistani government's attempts to work out peace deals with the local Taleban in the western tribal areas.