A US senator has said that an estimated 4,700 people have been killed in America's secretive drone war, the first time a government official has offered a total number of fatalities caused by nearly a decade of drone strikes, local media reported.

Republican senator Lindsey Graham, a staunch supporter of the drone raids, revealed the figure in a speech on Wednesday in his home state of South Carolina.

"We've killed 4,700," Graham was quoted as saying by the Easley Patch, a local website covering the small town of Easley. "Sometimes you hit innocent people, and I hate that, but we're at war, and we've taken out some very senior members of al-Qaeda," he told the local Rotary Club.

Graham's office did not dispute his reported remarks, but said that he had not divulged any classified information.

A spokesman told the AFP news agency that the senator "quoted the figure that has been publicly reported and disseminated on cable news."

US officials have sometimes hinted at estimates of civilian casualties, but never referred to an actual total body count.

"Now this is the first time a US official has put a total number on it," said Micah Zenko, a fellow at the Council on Foreign Relations.

'Tactical weapon'

If there was an official death toll estimate, it would be classified as secret, he added, raising the prospect that Graham could have broken secrecy laws.

Several organizations have tried to calculate how many militants and civilians may have been killed in drone strikes since 2004 but have arrived at a wide range of numbers.

The figure cited by Graham matches the high end of a tally by the London-based Bureau of Investigative Journalism. It says the number killed in drone strikes in Pakistan, Yemen and Somalia is between 3,072 and 4,756.

The Washington-based New America Foundation says there have been 350 US drone strikes since 2004, most of them during Barack Obama's presidency. And the foundation estimates the death toll at between 1,963 and 3,293, with 261 to 305 civilians killed.

US intelligence agencies and the White House have refused to divulge details about the strikes, which are officially termed classified, but officials have suggested that few if any civilians have been killed inadvertently.

In confirmation hearings this month for John Brennan, Obama's nominee to lead the CIA, senator Dianne Feinstein said she understood that the number of civilians killed was in the "single digits."

Despite criticism from lawmakers and rights advocates who have questioned the secrecy and the legality of the drone attacks, Graham defended Obama's reliance on the unmanned, robotic aircraft. "It's a weapon that needs to be used," Graham said. "It's a tactical weapon. A drone is an unmanned aerial vehicle that is now armed."

The Obama administration has insisted the "targeted killings" are "a last resort" against those plotting to attack the United States but who cannot be captured.

Opponents, however, say drone strikes amount to extrajudicial assassinations that sow resentment among local populations and lack oversight by Congress or courts.