KABUL, Afghanistan — American senators visiting Kabul on Thursday intensified pressure on President Hamid Karzai to sign a long-term security deal with the United States. And responding to a new crisis between the allies this week, they warned the Afghan government away from plans to summarily release dozens of detainees accused of attacking American forces.

"If these releases go ahead, it will do irreparable damage to the relationship," said Senator Lindsey Graham, Republican of South Carolina. "There will be a backlash in the U.S. Congress."

Mr. Graham and his fellow Republican, Senator John McCain of Arizona, have been frequent visitors to Afghanistan since well before the A United States invasion in 2001, and they are still among the most ardent congressional supporters of a continued American role in the country. But the two lawmakers, along with another visiting Republican, Senator John Barrasso of Wyoming, told reporters here on Thursday that they had expressed grave concerns to Mr. Karzai over a lunch meeting, warning that American will to stay involved with Afghanistan was dwindling, and that actions like the planned prisoner release were adding further strain.

American officials want the prisoners to be prosecuted and say the men are members of the Taliban or allied militant groups who are suspected in the deaths of 117 members of the Afghan or international security forces.

The men are being held near Bagram Air Base, in what was the main American prison in Afghanistan until it was transferred to the Afghan government eight months ago.

That transfer of authority over detainees seemed to have settled one of the most corrosive disputes between Mr. Karzai and his American backers, and it cleared the way for talks to secure a broader deal, known as the bilateral security agreement, that would allow for continued aid and a small American troop presence beyond 2014.

But the troop deal is now in limbo, derailed after its initial approval in November when Mr. Karzai said he would wait to sign it until after elections this coming spring. A long succession of American officials warned him that the delay could scuttle the entire deal, and billions of dollars in international aid. But Mr. Karzai then began issuing new demands, alienating some of his own Afghan supporters and infuriating American officials.

Now, the new flare-up over the potential prisoner release has revived the entire detainee issue, and the acrimony has escalated in a matter of days to the point where some of staunchest American supporters of a continued troop presence in Afghanistan are openly warning that the deal may collapse if the Afghan government frees the prisoners without hearings.

Mr. Graham suggested as much on Thursday, though he made no direct threats. "It will be devastating to any future negotiations with the United States," he said after the senators' meeting with Mr. Karzai.

Though Mr. Graham does not speak for the Obama administration, he and Mr. McCain are among the dwindling number of elected officials in Washington who are willing to advocate for keeping the United States engaged in Afghanistan. If their support were to cool, it would probably bolster the position of those inside and outside the administration pushing for a complete withdrawal when the NATO combat mission ends this year.

Mr. McCain appeared to acknowledge as much, saying he had told Mr. Karzai during lunch of a recent poll by CNN and ORC International that found only 17 percent of Americans support the war.

Still, Mr. McCain was publicly more measured on the prisoner release issue. In his comments, he focused on what he described as the narrowing differences over the security deal, and said of the prisoners: "We're going to have see what happens, how it happens and what the situation is before we make a judgment. We can't go any further in our comments."

The releases were ordered by a commission Mr. Karzai appointed to review the cases of those detained at the .

American officials believe freeing the men would be a violation of the prison deal struck March. They say Mr. Karzai personally assured them no prisoners implicated in attacks on NATO-led troops would be set free without a trial when Afghanistan took control of the prison.

"If this agreement is dishonored, how can you expect future agreements to be honored?" Mr. Graham said.

Figuring out where Mr. Karzai stands has proven more difficult. His office issued an ambiguous statement after his lunch with the American senators. "Afghanistan wants the innocent prisoners to be released based on Afghan laws, and criminals must be punished," it said.

The Afghan commission reviewing the cases has said the prisoners in question — American officials say there are 88, the Afghan commission says 86 — are innocent, or that there is not enough evidence to justify holding them until trial. The men are among the 650 detainees the commission has ordered freed since beginning its work last year, and the remaining ones could begin being freed as early as Saturday.

The commission has also ordered more than 100 other detainees to stand trial, a fact that commission members have cited as evidence they are willing to keep people suspected of being killers in prison.

In a statement, Mr. Karzai's office suggested that the Americans were being hypocritical on the issue of detainees. It was at the behest of American officials, after all, that a system of imprisoning battlefield detainees without trial was established in Afghanistan, over the criticism of many Afghan and international officials. And now, the palace said, the Americans were holding themselves up as protectors of Afghan justice.

"President Karzai stressed that many arrests have been made in violation of Afghan laws since the Bagram prison was built," said the statement from his office. "As result, a number of our innocent countrymen are imprisoned there."