The United States has reached a “critical moment” in its relationship with Pakistan following the killing of Osama bin Laden, Sen. John F. Kerry said Sunday from Kabul, before flying to Pakistan to address what he called “very disturbing” evidence of the Pakistani government’s knowledge of insurgent sanctuaries.

Kerry, chairman of the Senate Foreign Relations Committee, has emerged in the past few years as an important envoy for Afghanistan and Pakistan during times of crisis. Bin Laden’s killing this month in a military town in Pakistan has generated perhaps the most important crossroads yet: a chance for the United States to decide whether it will continue to work with a Pakistani government it suspects of supporting Taliban factions, reduce U.S. aid or make other changes to the relationship.

Kerry, in a news conference at the U.S. embassy in Kabul, did not tip the Obama administration’s hand on how it plans to proceed with Pakistan. But he said some of his congressional colleagues “have deep reservations about whether or not Pakistan is committed to the same goals, or prepared to be a full partner in pursuing these goals.”

“Yes, there are insurgents coming across the border, 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,” the Massachusetts Democrat said. Kerry said when he returns to Pakistan, “that will be, without any question, one of the subjects of conversation.”

“We are at a moment where we have to resolve some very serious issues,” Kerry said. “This is not a moment for anything except very sober, serious discussion with an understanding that there’s a lot at stake. There’s no other way to put it.”

Kerry, who traveled to the border town of Khost, as well as to the northern city of Mazar-e-Sharif, during his two-day visit to Afghanistan, also said he believes bin Laden’s death has added momentum to the process of reconciliation and potential for negotiations with the Taliban. Afghan officials told him the Taliban have shown “heightened interest” in negotiations since bin Laden’s death, but he didn’t go into specifics. He suggested the Taliban “could be a part of a different makeup of a new Afghanistan.”

He also focused his remarks on the prospect of accelerating transition to Afghan control and reducing the size of the American military presence in the country. He said U.S. aid money could be spent more effectively and “less could be spent in certain ways.”

“The footprint of our presence itself can be altered to become more effective and to see Afghans actually stepping out front more,” Kerry said.

In all, the death of bin Laden has raised the possibility of “the beginning of a new phase” in the Afghan war.