America's reliance on secretive drone missile strikes against terror suspects has set a "dangerous precedent" that could be imitated by other countries and trigger wider wars around the world, former senior US officials said in a report Thursday.

The ex-officials acknowledged that the robotic aircraft are a useful tool that is "here to stay," but urged President Barack Obama to lift the veil of secrecy that surrounds their use, introduce stricter rules for the strikes and take a hard look at whether the bombing raids were genuinely effective.

"The increasing use of lethal UAVs (unmanned aerial vehicles) may create a slippery slope leading to continual or wider wars," said the report by a bipartisan panel sponsored by the Stimson Center think tank.

The employment of drones for attacks outside of traditional battlefields "is likely to be imitated by other states as well," fueling instability and increasing "the risk of widening conflicts in regions around the globe," it said.

"US practices set a dangerous precedent that may be seized upon by other states -- not all of which are likely to behave as scrupulously as US officials," it said.

In the eyes of the rest of the world, the United States has essentially claimed the legal right to kill anyone it believes is a member of the Al-Qaeda network or its allies "in any state on earth, at any time, based on secret criteria and secret evidence," the report said.

The pervasive secrecy made it difficult for lawmakers in Congress to serve as a check on executive power and threatened to undermine traditional legal principles that underpin international law, it said.

Obama has promised to curb the secrecy around the strikes and in May said that any operation should not "create more enemies than we take off the battlefield."

The number of strikes in Pakistan and Yemen have decreased since 2010, according to unofficial tallies based on media reports, but the level of secrecy has changed little.

But the US president has faced criticism that he has failed to live up to his own pledges on drones to "uphold standards that reflect our values."

- Call to acknowledge strikes -

The report called on the Obama administration to adopt a more transparent stance and acknowledge drone strikes after they have been carried out in a foreign country. At the moment, US officials barely acknowledge the existence of the drone raids and do not reveal who was targeted and whether civilians were injured or how many killed.

"While secrecy may be required before and during each strike, strikes should generally be acknowledged by the United States after the fact," it said.

To ensure more accountability for a campaign largely conducted behind closed doors, the report urged Obama to create "a non-partisan independent commission to review lethal UAV policy."

The panel also said the Obama administration should fulfill its plan to transfer most of the drone strikes from the Central Intelligence Agency to the military, which operates under more transparent legal parameters compared to the spy service.

The authors questioned the overall efficacy of the drone strikes, saying it was not clear that the government had ever conducted a thorough analysis of the strategic advantages and disadvantages of using the robotic aircraft for counter-terrorism efforts.

It was time for the administration to conduct a "rigorous strategic review and cost-benefit analysis" of the drone raids, looking at the effect of past strikes on terror groups, local communities, public opinion and the cooperation of allies and partners, it said.

The ten-member task force that examined the controversial drone attacks included former senior intelligence and legal officials and was led by retired four-star general John Abizaid, who served as head of US Central Command, and Rosa Brooks, a former legal adviser at the Pentagon who is now a law professor at Georgetown University.

Human rights groups have long denounced the drone strikes in Pakistan and elsewhere as an unaccountable air war that operates virtually without scrutiny from Congress or the courts.

But the Stimson report was unusual as several of the authors were former high-ranking officials working in intelligence and counter-terrorism, including former legal advisers at the CIA, the State Department and the White House's National Security Council.