ISLAMABAD — A U.N. expert on Friday called on the United States to reveal the number of civilians it believes have been killed by American drone strikes targeting Islamic militants.

U.N. Special Rapporteur Ben Emmerson said that preliminary information gathered for a new report indicated more than 450 civilians may have been killed by drone strikes in Pakistan, Afghanistan and Yemen, but more work needs to be done to confirm the figures.

The U.S. provides very little public information about its drone program, especially in Pakistan and Yemen, where the CIA is involved in the attacks.

"The single greatest obstacle to an evaluation of the civilian impact of drone strikes is lack of transparency, which makes it extremely difficult to assess claims of precision targeting objectively," said the report, which was released Friday. The lack of transparency "creates an accountability vacuum and affects the ability of victims to seek redress."

The U.S. and other countries that use drones, such as Israel and the United Kingdom, have an obligation to investigate reported civilian casualties and clarify their legal justifications for the attacks, the report said.

The involvement of the CIA in the U.S. drone programs in Pakistan and Yemen "has created an almost insurmountable obstacle to transparency," the report said. Emmerson said he did not believe the U.S. could use national security considerations to justify withholding civilian casualty data.

The Pakistani government told the U.N. that at least 400 civilians have been killed by U.S. drone strikes in the country — a figure that Emmerson first revealed during a visit to Islamabad in March. The report said that the U.N. has confirmed more than 30 civilians were killed in drone attacks in Afghanistan in 2012 and 2013. Media reports indicate that at least 21 civilians have been killed in strikes in Yemen since 2011.

In total, Emmerson identified 33 drone attacks that appear to have resulted in civilian casualties, but he is still trying to confirm his findings with the states involved. He will present his final report to the U.N. Human Rights Council once he is finished.

Emmerson said in the report that drone strikes in Pakistan violate the country's sovereignty because the government has publicly objected to the attacks — a stance he first announced during his visit to Islamabad in March.

Others believed the situation with Pakistan is murkier since senior government and military officials have supported at least some of the strikes in the past, and U.S. officials say privately that some still do.

Pakistani Prime Minister Nawaz Sharif is scheduled to travel to Washington next week to meet with President Barack Obama. Drone strikes are expected to be on the agenda.