no-one knows...
Of the 600+ sightings reported every year, many of those are from America. The most well known is of course the 1947 crash sight in Rosewell, NM.
Seven sightings of UFO's in the US, Canada and France.
I have heard that there were sightings during both world wars, there could be more but those are the ones that I know about.
UFO hotspots include:Mexico City, southwest US, northern Europe.
So few sightings are actually reported, there are no reliable statistics.
A. The US government would have no reason to do that, since... B. If there were UFO sightings with abductions, they would not be under the government's control.
A large portion of UFO sightings occure in the US every year.
Except for a couple of UFO sightings I have not heard any US president communicating with aliens.
Of the 600 to 650 reported sightings (world wide) annually, there is no way to measure accurately which ones are more significant than the others. Many times, only in hindsight, do some aspects take on signifcant meanings. Some sightings are seen by many, these usually over urban areas. Some are sighted several nights/days in a row, some only a single sighting is reported. Only through researching each case individually will any important factors be revealed.
We will never get a straight or truthful answer from any branch of the US government.
According to MUFON, California, Texas and Florida, in that order, have the most reported sightings. All three of these states have many sunny days and clear nights which may make it easier to spot UFO's.
April 4, 1966, over Westfall High School, located in the suburban town Clayton South, Victoria, Australia. An interesting fact;there were two sightings on this exact date in the US.