The official number of civilians killed each year by U.S. drone strikes will remain unknown, after senators dropped a demand for a public declaration, congressional aides said, after the U.S. intelligence chief expressed concerns the disclosure might reveal classified information.

Drone strikes have been a key element in President Barack Obama’s battle plan against both Al-Qaeda and the Taliban, with unmanned aerial vehicles, often operated remotely from inside the United States, dropping bombs on targets in Afghanistan, northwest Pakistan, Yemen and Somalia.

But because of innocent bystanders that have been killed in the atttacks, the program has strained relations between the U.S. and governments in these countries and provoked public outrage.

The provision to reveal the number of civilian deaths was included in the main Senate intelligence bill for 2014, which passed a committee vote in November but has yet to be adopted in full. Right now, the U.S. government doesn't release casualty figures for drone strikes.

But Congressional aides said the reporting requirement has now been eliminated from the Senate bill. They spoke on condition of anonymity because they weren't authorized to speak publicly on the matter. It's unclear when the legislation may be sent to the Senate for a vote on its passage.

Director of National Intelligence James Clapper wrote a letter on April 18 to Dianne Feinstein, a Democrat and the Senate Intelligence Committee chairman, in which he expressed reservations with the bill's requirements.

"To be meaningful to the public, any report including the information described above would require context and be drafted carefully so as to protect against the disclosure of intelligence sources and methods or other classified information," Clapper said.

"We are confident that we can find a reporting structure that provides the American people additional information to inform their understanding of important government operations to protect our nation, while preserving the ability to continue those operations."

But some advocates have decried the change in the Senate version of the bill.

"When it comes to killing people, the administration's 'trust us' approach simply isn't good enough," said Zeke Johnson, director of Amnesty International USA's security and human rights program.

"Congress should be conducting vigorous oversight," he said. "Years after Obama's first strike, we're still in the dark about basic information, including the number of people killed, their names, and the legal memos used to justify the killings."

A House bill would still compel Obama to do some of those things, though its support is uncertain.

The legislation proposed by Reps. Adam Schiff, a Democrat, and Walter Jones, a Republican, would require Obama to produce an annual report with numbers of militants and civilians killed everywhere except in Afghanistan. The administration also would have to provide figures dating back to 2009.

Schiff said he was disappointed that the Senate dropped the requirement. "It is difficult for me to see how an annual tally of those combatants and noncombatants killed each year would reveal any meaningful information about sources and methods," he said.

Some organizations outside of the government have tried to keep an acounting of innocent civilians caught in the fire of America's drones.

The New America Foundation, for example, maintains a database of strikes using reports from major news media that rely on local officials and eyewitness accounts. It says at least 339 civilians have died in Pakistan and Yemen from U.S. drone attacks, most after President Obama took office in 2009. It labels a similar amount of the dead as "unknown" — neither militants nor noncombatants.

Pakistan's government told the United Nations last year that the approximately 2,200 people killed by drone strikes included at least 400 civilians.

Obama gave a speech announcing an overhaul of his administration's drone policy in May last year. Now, strikes are only said to target high-level terrorist targets who can't otherwise be captured, and when there is near certainty of no civilian casualties. U.S. drone attacks have dropped dramatically in recent months, especially in Pakistan.

But last week, Yemen's military, reportedly backed by U.S. drone strikes, hit a major Al-Qaeda base in southern Yemen and killed 55 people, including militants. A day earlier, a U.S. drone strike in southern Yemen killed at least nine suspected al-Qaeda militants and three civilians, Yemeni authorities said.

Al Jazeera and The Associated Press