WASHINGTON — Congressional Democrats on Thursday urged President Biden to overhaul his counterterrorism strategy and targeting criteria for drone strikes, citing grave concerns about “repeated civilian casualties arising from secretive and unaccountable lethal operations.”

The letter came a day after The New York Times published newly declassified surveillance footage providing additional details about the final minutes and aftermath of a botched drone strike in Kabul, Afghanistan, in August that killed 10 innocent civilians, including seven children. Eleven senators and 39 members of the House, led by Senators Elizabeth Warren of Massachusetts and Christopher S. Murphy of Connecticut, cited that strike as “emblematic of this systemic failure that has persisted across decades and administrations.”

“When there is little policy change or accountability for repeated mistakes this grave and this costly,” the senators wrote, “it sends a message throughout the U.S. armed forces and the entire U.S. government that civilian deaths — including deaths where there was no military target — are the inevitable consequence of modern conflict, rather than avoidable and damaging failures of policy.”

The letter, which was also led by Representative Ro Khanna of California, was a stinging rebuke of the administration’s current policies amid growing evidence of recurring episodes over multiple administrations in which civilian bystanders have been killed during drone strikes. And it came as top officials in Mr. Biden’s administration were working on a new policy governing drone warfare away from traditional battlefields.

“We cannot ignore the terrible consequences of U.S. drone strikes over several administrations,” Ms. Warren said in a statement. “I’ve long pushed for greater accountability for civilian casualties, and the president should seize this moment to systematically reform our counterterrorism strategy.”

Hours before lawmakers sent their letter to Mr. Biden, new reporting showed that a top-secret U.S. Special Operations unit struck Syria’s biggest dam using some of the largest conventional bombs in the U.S. arsenal, despite a military report warning not to bomb the dam because the damage could cause a flood that might kill tens of thousands of civilians.

The Defense Department has long said that it tries to minimize civilian casualties. But Defense Secretary Lloyd J. Austin III conceded in November that the military needed to do more to prevent them, days after an investigation by The Times revealed that top officers had sought to conceal a U.S. airstrike in Syria in 2019 that killed dozens of women and children.

Separate investigations, relying on the military’s own confidential assessments of more than 1,300 reports of civilian casualties obtained by The Times, showed that the air campaign against the Islamic State was marked by flawed intelligence, confirmation bias and scant accountability. Officials often dismissed allegations of civilian casualties with little evaluation, including failures to conduct simple internet searches.

“When U.S. strikes kill civilians abroad, it’s both a moral failure and national security liability,” Mr. Murphy said. “There’s no doubt Biden takes this issue more seriously than Trump, but we can and must do better. The U.S. should use force only lawfully and as a last resort, and when civilians die, there has to be accountability. That accountability simply has not been happening.”

Efforts by the Biden administration to recalibrate the nation’s policies governing drone strikes, in line with a broader effort by the president to wind down the war on terrorism, were complicated late this summer as Kabul fell to the Taliban, rendering the plans for Afghanistan obsolete. The process was meant to last only a few months, but after a year of drafts, deliberations and high-level meetings, it remains uncompleted.

Lawmakers, including Ms. Warren and Mr. Khanna, have previously pressed the Pentagon to account for significant undercounts of civilian casualties. And Congress approved a provision in this year’s defense policy bill requiring Pentagon officials to submit a number of reports, including on the department’s policies relating to civilian casualties resulting from U.S. military operations.

“The continuation of status quo policies that have flouted executive and congressional oversight and resulted in devastatingly high numbers of civilian casualties would run contrary to the Biden administration’s commitments to end our forever wars, and promote human rights and our core democratic values,” they wrote in Thursday’s letter.