By Declan Walsh and Mark Mazzetti for the New York Times.

LONDON — In a surprise move, Pakistan’s government on Wednesday sharply revised downward its official estimate of civilian casualties caused by American drone strikes in the tribal belt, highlighting again the contentious nature of statistics about the covert C.I.A. campaign.

The Ministry of Defense released figures to lawmakers saying that 67 civilians were among 2,227 people killed in 317 drone strikes since 2008. The remainder of those killed were Islamist militants, the ministry said.

The figures represented a civilian casualty rate of about 3 percent, falling far below earlier estimates from independent groups — and other government departments — that reported a rate of 6 percent and higher over the same period.

Recently, a United Nations special rapporteur on human rights and counterterrorism, Ben Emmerson, said that the Pakistani government had reported at least 400 civilian deaths since the drone campaign started in 2004.

In an email, Mr. Emmerson noted that the revised figures were “strikingly at odds” with those he had been given earlier by the Pakistani Foreign Ministry and said he would be writing to the government seeking clarification.

“It is essential that the government of Pakistan now clarify the true position,” he said.

In a twist, the latest figures place the Pakistani government on much the same page as the C.I.A., which Pakistan has repeatedly condemned for breaching its sovereignty in running the drone campaign.

Although the drone program remains classified and the Obama administration has never given an official estimate of civilian casualties, the C.I.A. and officials from other American agencies have insisted that the strikes have been very accurate and have killed only a small number of civilians.

And for all the angry denials from Pakistani military and civilian officials, a steady drip of news media leaks and other evidence suggests that they have quietly cooperated with at least some drone strikes.

But activists insist that civilian casualties remain a serious problem. The discrepancy in the various figures highlights both the opaque nature of the American campaign and the failure of the Pakistani authorities to properly investigate drone strikes, said Mustafa Qadri, a researcher with Amnesty International.

The American government should be “the first place to ask” about casualty figures, he said in an email. “If the strikes are precise and legal, then the U.S. should disclose the videos of the strikes and their full legal basis.”

A related problem, experts say, is trying to define civilians and casualties in a remote conflict zone like North Waziristan — particularly given that the international debate over the legality of drone strikes remains unresolved.

Ascertaining the truth about what happens in the tribal belt, a dangerous and lawless area along the Afghan border, is notoriously difficult. And new evidence from human rights groups, bolstered by witness testimony that was delivered by a Pakistani family in Washington this week, suggests that the drones are not as foolproof as American — and now some Pakistani — officials have suggested.

Last week, Amnesty International released a report, written by Mr. Qadri, that concluded that at least 19 civilians were killed in just two drone strikes in 2012, including a woman in North Waziristan who was picking vegetables in a field with her grandchildren.

But the new official Pakistani figures stated that no civilians were killed in North Waziristan in 2012 or this year.

The dead woman’s son and two grandchildren spoke about the attack at a news conference in Washington on Tuesday that was arranged by the Brave New Foundation, a liberal advocacy group that has funded a new documentary about the drone wars.

One grandchild, Nabila Rehman, 9, described seeing two missiles fired from a drone kill her grandmother as they worked together in the field. Speaking in Pashto through an interpreter, the girl also said she had suffered shrapnel wounds to her right hand.

She was accompanied by her father, a teacher, and her 13-year-old brother, Zubair ur-Rehman, who said that he preferred cloudy days when bad weather grounded the drone fleet.

“When the sky brightens and becomes blue, the drones return, and so does the fear,” he said.