KABUL, Afghanistan - U.S. Sen. John Kerry said on Sunday that the U.S. relationship with Pakistan was at a "critical moment" because of the killing of al-Qaida leader Osama bin Laden and that there were growing calls to cut aid to the country.

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.

Kerry said that "this is a critical moment in terms of the relationship with Pakistan" and said it was "fair to say that some of my colleagues in the House and Senate have deep reservations about whether or not Pakistan is committed to the same goals, or are prepared to be a full partner in pursing those goals."

He added that there were calls for a "shift in the aid program," and that "unless there is an improvement in the current situation, I think it will be very difficult to argue to the American people that while some programs are being cut, there ought to be additional funds allocated to the current confused situation."

Although he lauded Pakistani help in the past, including allowing American intelligence personnel to operate in Pakistan, -- which he said had helped the U.S. track bin Laden -- he also said he was concerned about some of their actions.

Kerry said that during a visit to Khost province, located on the eastern border with Pakistan, he was briefed on Taliban and insurgent safe havens just across the frontier. The safe havens, mostly in the province of North Waziristan, are used to launch attacks against American, coalition and Afghan troops. The Pakistani military has so far not gone after the safe havens.

"Yes there are insurgents coming across the border," Kerry said. "Yes, they are operating out of North Waziristan and other areas of the sanctuaries. And yes, there is some evidence of Pakistan government knowledge of some of these activities in ways that is very disturbing. That will be without any question one of the subjects of conversation."

But he also said that bin Laden's death may present a new opportunity for reconciliation with the Taliban in Afghanistan.

Kerry's trip also comes amid growing condemnation of the U.S. raid from Pakistani leaders, military officials, the Parliament and Islamic hard-liners. The Pakistani government has threatened reprisals, and the Parliament on Saturday passed a nonbinding resolution demanding a halt to drone strikes against militants in the tribal areas.

Pakistani say the drone attacks are a violation of their sovereignty and often kill civilians. The Parliament has threatened to stop U.S. and NATO supply convoys if the strikes continued.

A prominent hardline Islamist leader with suspected militant ties drew at least 4,000 people to a rally in the Pakistani city of Lahore on Sunday in support of bin Laden and against the U.S.

Hafiz Saeed, the leader of Jamaat-ud-Dawa, called bin Laden a martyr and demanded the Pakistani government break ties with the U.S. for killing him.

"Osama's death is the beginning of America's defeat," said Saeed, surrounded by several bearded gunmen and speaking from a makeshift stage in the heart of Lahore. "There is no justification for maintaining an alliance with America after its aggression against our country."

He said Pakistan's civilian leaders should give the military permission to shoot down U.S. drones, and called on Muslims all over the world to "stand up against America."

"Now is the start of a battle between Islam and infidels," said Saeed.

Jamaat-ud-Dawa is believed to be a front for the militant group Lashkar-e-Taiba, which is suspected of carrying out a series of attacks in the Indian city of Mumbai in 2008 that killed 166 people.