US intelligence officials have found no evidence confirming that unidentified flying objects encountered by US Navy pilots in recent years were alien spacecraft but also have not reached a definitive assessment as to what these mysterious objects might be, according to five sources familiar with the findings of an upcoming report on UFOs that is expected to be delivered to Congress later this month.

According to three of those sources, the report does not however rule out the possibility they are alien spacecraft.

While that uncertainty is likely a blow to the hopes of UFO enthusiasts who were hoping for definitive proof of extraterrestrial life, it does not minimize the significance of the report, especially given what sources describe as a years-long battle inside the Pentagon over whether even to acknowledge what are now hundreds of unexplained sightings by US military personnel.

The New York Times was first to publish details of the upcoming report.

US officials also cannot rule out the possibility that these flying objects were aircraft belonging to American adversaries, namely Russia and China -- a potentially more troublesome conclusion that raises a host of potential national security concerns, one of the sources said.

However, the forthcoming report is expected to conclude that the objects are not secret American technology, the source added.

Historic UFO report

For the first time, the US government is preparing to issue an extraordinary unclassified report detailing what it knows about a series mysterious encounters with unidentified flying objects, or what are more commonly known as UFOs.

While the report isn't expected to confirm the existence of extraterrestrial life, the mere fact that the intelligence community is set to acknowledge these incidents at all represents a remarkable shift in the way US officials think about this phenomenon.

The US government has long been reluctant to disclose any information related to numerous reported sightings of unidentified flying objects encountered by military pilots in restricted airspace. Multiple sources told CNN that for years the Defense Department has tried to avoid committing too many resources to what still amounts to a fringe priority.

But a steady drumbeat of pressure from lawmakers has now forced the Pentagon and the US intelligence community to acknowledge an uncomfortable truth: these encounters represent a potential security threat that simply cannot be explained.

"The sense I have from the briefings I've sat in on is nobody knows what to do about it. There's not a policy prescription and that's why there is such a hesitancy to deal with the issue," Rep. Michael Waltz, a Republican member of the House Armed Services Committee, told CNN in a recent interview.

"But it's disturbing either way, you know, is this alien life or do adversaries or someone else have capabilities that far exceed anything we have the ability to understand track or counter? Either way it's really bad," said Waltz.

No proof of little green men

The upcoming report is unlikely to satisfy UFO-ologists who support what's known as "Disclosure," or the government release of definitive evidence that it has been contacted by extraterrestrial life.

"Most people would be hoping for visual evidence of little green men, which is almost certainly not going to be the case," a congressional aide told CNN, referring to expectations about the report's contents.

In fact, multiple sources told CNN that they don't expect the intelligence community to release much specific information at all, partly because if these strange sightings are actually next-gen technology fielded by a foreign adversary, intelligence officials won't want to tip them off on what the US has seen.

Additionally, technical intelligence experts still need more information on some of these encounters to reach a conclusive assessment, an administration official involved in the current investigative efforts said. But the same official also told CNN that many encounters turn out to have perfectly normal explanations, like a weather balloon or a drone. In some instances, sightings have been removed from the US government database tracking these encounters because officials were able to explain them.

Yet multiple unexplainable reports from pilots in recent years led to tension within the Pentagon about how much time and resources to devote to investigating these incidents, according to sources and documents reviewed by CNN.

They also fueled a desire on Capitol Hill for more information and the belief among several key lawmakers that Congress must flex its oversight powers to ensure the issue is being handled appropriately. Requiring production of the upcoming UFO report is one way lawmakers have signaled that they intend to use that authority, sources say.

"One of the functions of a course like this is that it forces actual coordination within the agencies and makes clear that Congress is actually serious about its oversight function and that there's going to be increased scrutiny along the way," the congressional aide said. "Some of it is a product of getting the agencies to take the issue more seriously and trying to help get rid of the stigma surrounding it."