Kerry, chairman of the powerful Senate Foreign Relations Committee, said that although Pakistan had in the past sacrificed much in the battle against al-Qaida and its own domestic Islamic insurgency, the killing of bin Laden by U.S. Navy SEALs near the capital had raised questions.

Pakistan was not told about the raid in the city of Abbottabad until after it was completed. Some in the United States were critical of the Pakistan's security forces for having failed to detect the terror leader — or worse, giving him protection.

"We are at a moment where we have to resolve some very serious issues. This is not a moment for anything except very sober, serious discussion with an understanding that there is a lot at stake, there is no other way to put it. I think they understand that, we understand that," Kerry said in the Afghan capital.

He was to travel to Pakistan later Sunday for meetings with government and military officials. Kerry spent two days in Afghanistan meeting with Afghan President Hamid Karzai, his Cabinet and U.S. military officials prosecuting the campaign against the insurgency.