Defense Secretary Robert Gates said Thursday he has "real concern" about a sharp rise in attacks by insurgent forces in eastern Afghanistan and says it reflects infiltration of fighters from Pakistan.

Gates was asked at a Pentagon news conference what he thought of a report by a senior U.S. general in Afghanistan on Tuesday that insurgent attacks in the east have increased by 40 percent this year.

"It is a matter of concern — real concern," Gates replied.

"It's an issue that clearly we have to pursue with the Pakistani government," he added.

The defense secretary said one reason for the jump in insurgent attacks in that part of Afghanistan is that fighters have been able to cross the border without facing sufficient pressure by Pakistani troops.

"It actually was not bad until a few months ago," he said, when the Pakistani government began negotiating peace or ceasefire deals with a variety of militant groups in areas bordering Afghanistan.

"The pressure was taken off these people," as a result of such deals, he added. And that has meant fighters are "now more free to cross the border and create problems for us," Gates said.