The Pakistani army is facing fresh accusations of carrying out extra-judicial killings and torture, claims which could threaten US funding for any units singled out for abuse.
New York-based Human Rights Watch said it had briefed US State Department and congressional officials about mounting evidence of more than 200 summary executions in Swat Valley in the past eight months of suspected Taliban sympathisers.
Pakistan's army denied the group's accusations of abuse in Swat, home to about 1.3 million people and the site of a much-lauded military operation last year to take back the former Taliban stronghold.
"Swat is open to journalists and you can conduct investigative reporting there," Pakistani Army spokesman Major General Athar Abbas told Reuters in Islamabad. "Have you seen any sort of report in Pakistani newspapers?"
The Lahore-based Human Rights Commission of Pakistan provided a list of 249 suspected extra-judicial killings from July 30, 2009, to March 22, 2010, saying most of the bodies were found in Swat. It said independent journalists and locals widely believed security forces were behind them.
Officials in Washington said they were taking the accusations of abuse seriously. The Obama administration has raised the matter with Islamabad, officials said.
"We have shared our concern about these allegations with senior Pakistani officials and will continue to monitor the situation closely," State Department spokesman PJ Crowley said.
Defense Secretary Robert Gates has also discussed US concerns with Pakistani military and government officials.
"While our strong bilateral relationship with Pakistan and our close partnership in combating terrorism are very important to us, we take allegations of human rights abuses seriously," said Geoff Morrell, Pentagon press secretary.
White House National Security Council spokesman Mike Hammer said "we are seeing positive forward motion from our friends" in Pakistan on the issue, but did not elaborate.
Accusations of rights abuses by the Pakistani military are not new, but the latest allegations come at a highly sensitive moment for US-Pakistan relations.
Washington, which faces frequent criticism in Pakistan following suspected CIA drone strikes on militants, wants to strengthen ties with Islamabad. It also wants to encourage more operations against Islamic extremists following the Pakistani military's success in Swat and also in South Waziristan.
But Tom Malinowski, Washington advocacy director for Human Rights Watch, said the pace of extra-judicial killings in Pakistan was "not slowing down."
The United States is obliged to enforce a law authored by Senator Patrick Leahy banning assistance to foreign military units facing credible accusations of abuses, he said.
"If they obtain or receive credible information that a particular unit is engaged in this kind of behavior, they have to de-fund the unit," Malinowski said.
Human Rights Watch is not yet able to single out any units for the abuses, which also include illegal detention, he said.
Since the September 11, 2001, attacks on the United States, Washington has given more than $15 billion in aid to Pakistan, most of it for security-related work.
The Pentagon's Morrell said aid to Pakistan's military had not been cut off. He said there had been productive dialogue with Islamabad "about how we can help them build their capacity to deal with detainees in a rule of law framework."
"This work has been going on for several months now and we are pleased to see progress being made," he said.
The State Department said US aid was being delivered in full accordance with US law, and added that assistance to Pakistani security forces incorporated human rights training.
Human Rights Watch said the Army was targeting civilians who had voiced support for the Taliban when they controlled Swat or were suspected of providing them food or shelter.
"People are taken away, and sometimes they turn up a few days or weeks later having been tortured. Sometimes they disappear. Sometimes their body is dumped with a bullet in the head," Malinowski said.
He also described cases of illegal detention.
"A son has gone off to fight with the Taliban, and so another son is taken as a hostage," he said. "And the father is told: We will release son No. 2 when son No. 1 turns himself in."
He said such abuses ran against US counter-insurgency strategy and could erode support for Pakistan's government. The Army remains popular in Swat, which endured a brutal Taliban rule that included public beheadings and floggings.
The White House National Security Council's Hammer said the Obama administration had briefed Congress on the allegations.
Leahy's office declined to comment on the specific allegations of abuse but called for enforcement of US law "so US aid does not go to army units that violate human rights."
"And Pakistani authorities need to know how US law is applied," spokesman David Carle said.